CI improvements

[ptomato]

Migrate linter and test generation workflow to GitHub Actions, and add some improvements

• Faster, with parallelized jobs
• Don't run unit tests of tools if the tools didn't change
• Now adds annotations to the PR for linter errors, so that contributors will more easily notice, locate, and fix them

To merge this, we'll have to go into the settings, set the "ci/circleci: Test262: verify tools; build & lint tests" workflow to non-required, merge it, and then go back to the settings and set whichever new workflows we'd like to require.

[ptomato]

Screenshot of what the linter annotations look like:

[ptomato]

Hopefully everyone's happy with the current state of this.

@ljharb would you mind doing the honors with the settings? You'll have to set "ci/circleci: Test262: verify tools; build & lint tests" to non-required, merge the PR, then set "Checks / Lint tests" and "Checks / Build generated tests" to required. I think it's under "Code and automation → Branches" in the settings. (https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-protected-branches/managing-a-branch-protection-rule)

[ljharb]

All done. Merge at wil.

[ptomato]

Merged!