GitHub Actions: Add task to comment on PRs with gutenberg.run link.

[iandunn]

Description

This adds a new workflow that will create a comment on new PRs, with a link to a gutenberg.run preview site.

Gutenberg.run automatically creates functioning WordPress sites that run the version of Gutenberg from a PR's branch. The sites are accessible to anybody, making testing much easier for many people.

For example, http://gutenberg.run/30255

How has this been tested?

I committed these changes to trunk on my fork, and then opened test PRs against my fork. It'd probably also work to open a test PR against https://github.com/iandunn/gutenberg/tree/add/pr-preview-site-workflow.

I also created a unit test.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code has proper inline documentation.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[aduth]

Maybe it's changed or I'm imagining things, but I seem to recall the artifact build would only occur on pull requests with code changes? It looks like it's running on all pull requests now, so I'd guess it's not a problem to always comment on every pull request.

Since the build could take longer to finish and trying to load a gutenberg.run site before the build is complete would result in an error, it'd be nice if there was an easy way to wait to drop the comment until after the build is complete. Thinking if the automation is super fast enough to leave the comment that it appears right after the author submits the pull request, they might be led to click it immediately.

Alternatively, we can work to make gutenberg.run more durable to the "not yet ready" cases 😅

[iandunn]

wait to drop the comment until after the build is complete

🤔 , I thought it was running after that task, but may have overlooked that, or made some changes to my fork that messed it up. I'll look into it.

[iandunn]

You're right, it does run before the build, which is bad.

It seems like it should be possible to make it depend on the build workflow. I'm working on that, but testing is difficult.

[annezazu]

We're in more of a WordPress Playground world at this point so closing out this PR in favor of efforts to add playground preview links: #60795