Replace .npmignore files, ignore all test/fixture/docs directories (#50784)

Summary:
Pull Request resolved: #50784

Migrates us from several `.npmignore` files to `package.json#files`, for the main `react-native` package.

This reduces the size of our npm package by 38 files — in particular, catching previously missed `__tests__` and `__docs__` directories.

```diff
- npm notice package size: 4.4 MB
- npm notice unpacked size: 23.7 MB
- npm notice total files: 4533
+ npm notice package size: 4.4 MB
+ npm notice unpacked size: 23.7 MB
+ npm notice total files: 4533
```

NOTE: `"files"` or `.npmignore` — one or the other!
- Having `.npmignore` at the package root does not behave(!). Having `"files"` as one source of truth is better and safer. See npm/cli#6221.

Changelog: [Internal]

Reviewed By: hoxyq

Differential Revision: D73185321

fbshipit-source-id: 429c9fec4f447d63440a38bb8e3f3ce3bd155414

Author: huntie

packages/react-native/Libraries/.npmignore

@@ -49,48 +49,61 @@
     "React",
     "!React/Fabric/RCTThirdPartyFabricComponentsProvider.*",
     "ReactAndroid",
+    "!ReactAndroid/.cxx",
+    "!ReactAndroid/build",
+    "!ReactAndroid/external-artifacts/artifacts",
+    "!ReactAndroid/external-artifacts/build",
+    "!ReactAndroid/hermes-engine/.cxx",
+    "!ReactAndroid/hermes-engine/build",
+    "!ReactAndroid/src/main/third-party",
+    "!ReactAndroid/src/test",
     "ReactApple",
     "ReactCommon",
     "README.md",
     "rn-get-polyfills.js",
+    "scripts/bundle.js",
+    "scripts/cocoapods",
+    "scripts/codegen",
     "scripts/compose-source-maps.js",
     "scripts/find-node-for-xcode.sh",
-    "scripts/bundle.js",
     "scripts/generate-codegen-artifacts.js",
     "scripts/generate-provider-cli.js",
     "scripts/generate-specs-cli.js",
-    "scripts/codegen",
-    "!scripts/codegen/__tests__",
-    "!scripts/codegen/__test_fixtures__",
     "scripts/hermes/hermes-utils.js",
     "scripts/hermes/prepare-hermes-for-build.js",
     "scripts/ios-configure-glog.sh",
-    "scripts/xcode/ccache-clang++.sh",
-    "scripts/xcode/ccache-clang.sh",
-    "scripts/xcode/ccache.conf",
-    "scripts/xcode/with-environment.sh",
     "scripts/native_modules.rb",
     "scripts/node-binary.sh",
-    "scripts/packager.sh",
     "scripts/packager-reporter.js",
+    "scripts/packager.sh",
     "scripts/react_native_pods_utils/script_phases.rb",
     "scripts/react_native_pods_utils/script_phases.sh",
     "scripts/react_native_pods.rb",
-    "scripts/cocoapods",
-    "!scripts/cocoapods/__tests__",
     "scripts/react-native-xcode.sh",
+    "scripts/xcode/ccache-clang.sh",
+    "scripts/xcode/ccache-clang++.sh",
+    "scripts/xcode/ccache.conf",
+    "scripts/xcode/with-environment.sh",
     "sdks/.hermesversion",
     "sdks/hermes-engine",
     "sdks/hermesc",
     "settings.gradle.kts",
     "src",
+    "!src/private/testing",
     "template.config.js",
     "template",
     "!template/node_modules",
     "!template/package-lock.json",
     "!template/yarn.lock",
     "third-party-podspecs",
-    "types"
+    "types",
+    "!**/__docs__/**",
+    "!**/__fixtures__/**",
+    "!**/__flowtests__/**",
+    "!**/__mocks__/**",
+    "!**/__test_fixtures__/**",
+    "!**/__tests__/**",
+    "!**/__typetests__/**"
   ],
   "scripts": {
     "prepack": "node ./scripts/prepack.js",