Tests: add support for latest and latest-\d+ in test runner

[timmywil]

Adds support for filtering browser version by "latest" and "latest-n", which we can now use in the browserstack.yml.

I also made it so npm run test:unit -- --list-browsers=_latest lists the latest version of all browsers, rather than matching on the last browser in the sorted list.

I considered adding support for "latest" to OS versions, but didn't want to spend too much time on it since OS versions don't get updated nearly as often as browser versions.

I looked into setting browser versions using the environment, but workflows do not support environment variables at the matrix level, only in steps. There were a couple ways around that, but they were convoluted.

• also fix an issue with retries. It was starting each retry the first time, but closing the worker immediately afterward, so any failures were getting ignored. Fortunately, retries were not actually common and we still have all green: https://github.com/timmywil/jquery/actions/runs/8075030746. In the process, I edited the CLI documentation for --retries to make it clear it only applies on BrowserStack runs.

Checklist

• [x] New tests have been added to show the fix or feature works
• [x] If needed, a docs issue/PR was created at https://github.com/jquery/api.jquery.com

[timmywil]

Closing in favor of another upcoming PR that brings in all the improvements from the 3.x PR. I should have the PR feedback from this one in both of those PRs.