Statistics: add GitHub Actions workflow

[ddbeck]

This adds a GitHub Actions workflow that posts a comment with the results of stats-report.ts to a designated issue based on a nominated pair of commits. By itself, this won't be terribly useful. In a follow up PR, I'll extend this to post on a schedule, like I previously did manually in #788.

It's GitHub Actions, so this is tough to review. Mostly, I suggest looking at the run steps to make sure I'm not doing anything confusing or destructive. I imagine "add a comment to explain this" as the most likely thing you'll find here.

If you really want to test this, you can push this branch to your own fork, then run a command like:

YOU="your_github_username"
ISSUE="an issue number goes here"
gh workflow run -R $YOU/web-features post_stats.yml --ref=statistics-add-github-actions-workflow-for-produ/vxuyrykmprkx -f issue_number=$ISSUE -f previous=(git rev-parse HEAD~) -f current=(git rev-parse HEAD)

Or you could just trust me that I've done this a dozen times myself. 😓

Depends on #4294.

Towards #3510.