Complete migration to GitHub Actions.

See gh-411

Closes gh-411

Closes gh-419

Closes gh-418

Previously, the check would apply to interfaces and annotations as
well. This commit tightens up the check so that it only applies to
classes.

Closes gh-364

Closes gh-281

Bumps [xml2js](https://github.com/Leonidas-from-XIV/node-xml2js) to
0.5.0 and updates ancestor dependency
[@vscode/vsce](https://github.com/Microsoft/vsce). These dependencies
need to be updated together.

Updates `xml2js` from 0.4.23 to 0.5.0
- [Commits](https://github.com/Leonidas-from-XIV/node-xml2js/commits/0.5.0)

Updates `@vscode/vsce` from 2.17.0 to 2.19.0
- [Release notes](https://github.com/Microsoft/vsce/releases)
- [Commits](microsoft/vscode-vsce@v2.17.0...v2.19.0)

---
updated-dependencies:
- dependency-name: xml2js
  dependency-type: indirect
- dependency-name: "@vscode/vsce"
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

See gh-375

* gh-375:
  Bump xml2js to 0.5.0 and @vscode/vsce to 2.19.0

Closes gh-375

Closes gh-420

 - Clear the new fields each time a new tree is begun. This prevents
   state from one tree affecting the checking of another.
 - Allow abstract test classes to be public

Closes gh-281

Closes gh-421