add CI for publishing on the fortran-lang twitter account upon merging a certain PR

.github/workflows/gen_tweet.yaml

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+name: twitter bot
+
+# runs on issues that have been created
+on:
+  issue_comment:
+    types: [created]
+
+# setup environment variables
+env:
+  TWEET_KEYWORD: '__[tweet]__'
+  UNTWEET_EDIT_COMMENT_KEYWORD: '__[tweet withdrawn]__'
+  TWEET_MAX_CHAR: 280
+  TWITTER_HANDLE: fortranlang
+  TWITTER_URL: https://twitter.com/fortranlang
+
+jobs:
+  # withdraws the tweet CI comment
+  untweet:
+    runs-on: ubuntu-latest
+    if: github.event.issue.pull_request && startsWith(github.event.comment.body,'#untweet')
+    steps:
+    - name: find the tweet comment matching the keyword env.TWEET_KEYWORD
+      uses: peter-evans/find-comment@v1
+      id: fc
+      with:
+        issue-number: ${{github.event.issue.number}}
+        comment-author: github-actions[bot]
+        body-includes: ${{env.TWEET_KEYWORD}}
+#   - run: echo ${{steps.fc.outputs.comment-id}}
+    - name: if tweet CI comment is found, replace CI comment to undo it
+      if: ${{steps.fc.outputs.comment-id != null}}
+      uses: peter-evans/create-or-update-comment@v1
+      with:
+        comment-id: ${{steps.fc.outputs.comment-id}}
+        edit-mode: replace
+        body: |
+          ${{env.UNTWEET_EDIT_COMMENT_KEYWORD}}
+    - name: fetch comment url
+      if: ${{steps.fc.outputs.comment-id != null}}
+      id: get-comment-url
+      run: |
+        echo ::set-output name=comment-url::$(echo "${{github.event.issue.pull_request.url}}\#issuecomment-${{steps.fc.outputs.comment-id}}" | sed 's/api\.//; s/repos\///; s/pulls/pull/')
+    - name: if tweet CI comment found, report it in a new comment
+      if: ${{steps.fc.outputs.comment-id != null}}
+      uses: peter-evans/create-or-update-comment@v1
+      with:
+        issue-number: ${{github.event.issue.number}}
+        body: |
+          ${{env.UNTWEET_KEYWORD}} removed previous tweet [above](${{steps.get-comment-url.outputs.comment-url}}).
+  # writes a tweet CI comment
+  tweet:
+    runs-on: ubuntu-latest
+    if: github.event.issue.pull_request && startsWith(github.event.comment.body,'#tweet')
+    steps:
+    - name: search for a previous tweet CI comment matching the keyword env.TWEET_KEYWORD
+      uses: peter-evans/find-comment@v1
+      id: fc
+      with:
+        issue-number: ${{github.event.issue.number}}
+        comment-author: github-actions[bot]
+        body-includes: ${{env.TWEET_KEYWORD}}
+#   - run: echo ${{steps.fc.outputs.comment-id}}
+    - name: if tweet CI comment is found, replace CI comment to undo it
+      if: ${{steps.fc.outputs.comment-id != null}}
+      uses: peter-evans/create-or-update-comment@v1
+      with:
+        comment-id: ${{steps.fc.outputs.comment-id}}
+        edit-mode: replace
+        body: |
+          ${{env.UNTWEET_EDIT_COMMENT_KEYWORD}}
+#   - name: fetch comment url
+#     if: ${{steps.fc.outputs.comment-id != null}}
+#     id: get-comment-url
+#     run: |
+#       echo ::set-output name=comment-url::$(echo "${{github.event.issue.pull_request.url}}\#issuecomment-${{steps.fc.outputs.comment-id}}" | sed 's/api\.//; s/repos\///')
+#   - name: if tweet CI comment is found, report it in a new comment
+#     if: ${{steps.fc.outputs.comment-id != null}}
+#     uses: peter-evans/create-or-update-comment@v1
+#     with:
+#       issue-number: ${{github.event.issue.number}}
+#       body: |
+#         ${{env.UNTWEET_KEYWORD}} removed previous tweet [above](${{steps.get-comment-url.outputs.comment-url}}).
+    - name: extract the new tweet message
+      id: get-comment-body
+      run: |
+        body=$(echo "${{github.event.comment.body}}" | sed '1 s/#tweet//' | sed '1 s/ //')
+        echo ::set-output name=tweet_length::$(echo $body | tr -d '\n\r%' | wc -c)
+        body_quote=$(echo $body | sed 's/^/> /')
+        # escape string content to preserve new lines
+        # (see example in https://github.com/peter-evans/create-or-update-comment)
+        body="${body//'%'/'%25'}"
+        body="${body//$'\n'/'%0A'}"
+        body="${body//$'\r'/'%0D'}"
+        body_quote="${body_quote//'%'/'%25'}"
+        body_quote="${body_quote//$'\n'/'%0A'}"
+        body_quote="${body_quote//$'\r'/'%0D'}"
+        echo ::set-output name=body::$body
+        echo ::set-output name=body_quote::$body_quote
+    - name: print new tweet CI message in a new comment
+      uses: peter-evans/create-or-update-comment@v1
+      with:
+        issue-number: ${{github.event.issue.number}}
+        body: |
+          ${{env.TWEET_KEYWORD}} the following message will be tweeted in [@${{env.TWITTER_HANDLE}}](${{env.TWITTER_URL}}) after this PR is merged (character count: ${{steps.get-comment-body.outputs.tweet_length}}/${{env.TWEET_MAX_CHAR}}):
+          ${{steps.get-comment-body.outputs.body_quote}}

.github/workflows/send_tweet.yaml

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+name: send a tweet if PR is merged (and there is a tweet to be sent)
+
+on:
+  pull_request:
+    types: [closed]
+env:
+  TWEET_KEYWORD: '__[tweet]__'
+  TWEET_SENT_KEYWORD: '__[tweet sent]__'
+jobs:
+  publish_tweet:
+    runs-on: ubuntu-latest
+    if: github.event.pull_request.merged
+    steps:
+    - name: find the tweet comment matching the keyword env.TWEET_KEYWORD
+      uses: peter-evans/find-comment@v1
+      id: fc
+      with:
+        issue-number: ${{github.event.pull_request.number}}
+        comment-author: github-actions[bot]
+        body-includes: ${{env.TWEET_KEYWORD}}
+    - name: if tweet CI comment is found, extract tweet from comment body
+      id: extract-tweet
+      if: ${{steps.fc.outputs.comment-id != null}}
+      run: |
+        body=$(echo "${{steps.fc.outputs.comment-body}}" | sed '1d' | sed 's/> //')
+        # escape file content to preserve new lines
+        # (see example in https://github.com/peter-evans/create-or-update-comment)
+        body="${body//'%'/'%25'}"
+        body="${body//$'\n'/'%0A'}"
+        body="${body//$'\r'/'%0D'}"
+        echo ::set-output name=body::$body
+    - name: tweet the message
+      if: ${{steps.fc.outputs.comment-id != null}}
+      uses: ethomson/send-tweet-action@v1
+      with:
+        status: ${{steps.extract-tweet.outputs.body}}
+        consumer-key: ${{secrets.TWITTER_API_KEY}}
+        consumer-secret: ${{secrets.TWITTER_API_SECRET_KEY}}
+        access-token: ${{secrets.TWITTER_ACCESS_TOKEN}}
+        access-token-secret: ${{secrets.TWITTER_ACCESS_TOKEN_SECRET}}
+    - name: clear tweet keyword
+      if: ${{steps.fc.outputs.comment-id != null}}
+      uses: peter-evans/create-or-update-comment@v1
+      with:
+        comment-id: ${{steps.fc.outputs.comment-id}}
+        edit-mode: replace
+        body: ${{env.TWEET_SENT_KEYWORD}}