Imports the existing Linux implementation from the flutter-desktop-embedding repository, with very minor changes for the move:

• Updated the repo metadata (issue link, repository link).
• Reordered pubspec.yaml to match repo conventions.
• Updated the READMEs to match repo standards.
  • This change says it's endorsed even though it's not yet because as soon as this lands I'll do a follow-up to endorse it; doing it up-front avoids an extra PR to change it later (or forgetting to do so; see below).
• Ran the autoformatter.
• Added missing copyrights in generated code, and year-standardized the existing copyrights to match the repo.
• Updated the LICENSE file to match repo standard.
• Added AUTHORS.
• Bumped the version to release the updated version.

Adds xvfb to Linux unit tests, since the file_selector test requires GTK initialization that fails without a display server.

This also opportunistically makes some minor non-code cleanups to the Windows and macOS implementations, noticed while I was making sure everything was consistent:

• Updates their READMEs to indicate that they are endorsed, which I apparently never did.
• Moves flutter_test to dev_dependencies, rather than accidentally being in dependencies. (In practice, not a big deal since it's from the SDK, but there's no reason for it to be in the non-dev section.)

Part of flutter/flutter#70221

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.