[url_launcher_linux] Add Linux url_launcher plugin

[robert-ancell]

Description

Add Linux url_launcher plugin.

Related Issues

flutter/flutter#41721

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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Breaking Change

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[stuartmorgan-g]

This should have an example and test, as with all the other plugins in flutter/plugins. And we can actually check in the full example app now for Linux, since it's stabilized!

[robert-ancell]

This should have an example and test, as with all the other plugins in flutter/plugins.

cough url_launcher_macos cough 👀

[stuartmorgan-g]

This should have an example and test, as with all the other plugins in flutter/plugins.

cough url_launcher_macos cough 👀

https://github.com/flutter/plugins/blob/master/packages/url_launcher/url_launcher_macos/example/test_driver/

Note that https://cirrus-ci.com/task/6609503187632128 isn't showing any test having run for this package, which is the problem. There should be an example, which the existing test script can run with flutter driver. url_launcher_macos has that.

[stuartmorgan-g]

Added tests that should be driven by CI. That found:

• The example didn't compile, because the CMake project name didn't match the plugin name (url_launcher vs url_launcher_linux)
• The return value for canLaunch for a bogus URL didn't match the existing mobile test expectations (an error rather than false), so I changed that for consistent behavior with existing code.

[stuartmorgan-g]

Still one test failure, which CI isn't picking up (filed flutter/flutter#60943).

I suspect the failure means there's no browser installed on the image, so trying a run with Chromium added to the Dockerfile.

[stuartmorgan-g]

Test is now passing after setting up the Docker image correctly. Added the iOS stub, which I just noticed was missing. Assuming everything is green I'll land this tomorrow.

[stuartmorgan-g]

Published; PR to endorse it will follow.