(chore) Fix build size report

[bradleymackey]

• Fixes the size compression report in (chore) add gzip size compression report #3400 by using GitHub's recommended workflow for posting comments on PR originating from forks from GitHub Actions. The steps for this workflow are outlined in this blog post.
  • In summary, pull_request does not have repo write permissions on the repository (so it can't leave a comment on incoming PRs from forks). pull_request_target does, but is unsafe to use as an event for checking-out untrusted PRs from forks. We should use pull_request followed by workflow_run to post the comment to the PR.
• The first (pull_request) step will run npm run build-cdn on the base and the PR, then generate the markdown report which is saved as an artifact to the GitHub Actions service. This triggers the second (workflow_run) step, which reads this report and posts it as a comment (without checking-out or running untrusted code from a fork).
• The workflow_run will only be run for PRs targeting the base branch (main).
• Computes gzip size for all minified {js,css} files in the build directory.
• Leaves a new comment each time the workflow is run.

Changes

• Remove old size compression report workflow, as it couldn't comment on PRs from forks.
• Create new size report script for comparing 2 build directories, outputting markdown.
• Create 2 new GitHub Actions workflows for building and commenting the size report on PRs.
  • Test/demo here. Example with changes:

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Checklist

• Added markup tests this is a CI change only
• Updated the changelog at CHANGES.md

[joshgoebel]

Let me know when you're done here. :) Looks pretty awesome.

[bradleymackey]

@joshgoebel I think it's ready now! A last possible change is whether or not we want only 1 sticky comment that gets edited each time changes are pushed, or a new comment each time (current behaviour).

[joshgoebel]

I think new comment is good.