Use GH Actions for CI

[cwillisf]

Resolves

Resolves ENG-46

Proposed Changes

Implement GitHub Actions CI for Scratch Link and remove CCI config. The actual build steps are extracted into reusable actions to make the main ci.yml more DRY.

Supporting changes:

• Delete the maximum effort install script for Visual Studio Mac since GHA Mac images come with it preinstalled.
• Allow Xcode to make some adjustments to the macOS project files. I don't fully understand the changes it made, but it made the build work, so...

Note that the calculated version number looks incorrect; I believe that's a limitation of using GitVersion for that. I might switch the build-time version calculation to use the value from the package.json file, but that'll be a separate PR.

[cwillisf]

Sorry, I forgot to finish the macOS build steps (notarization, etc.) so this isn't really ready for review. The option to convert this PR to a draft isn't showing up for me, so I'll remove the review requests for now. I'll re-request when it's ready.