Obviously it's not yet possible to just migrate `gulp browsertest` to GitHub Actions, however it's already possible to at least run the browser tests there which allows collection of more code coverage data.
This should thus give us more realistic coverage numbers, since currently there's many `src/` files that have very low code coverage.
By taking advantage of the fact that the GitHub Actions runners provide multiple cores, these tests are also fairly fast:
- The ubuntu-latest/firefox job complete in ~9 minutes.
