Set up a GitHub Actions workflow

[phenaproxima]

This project will be tested using GitHub Actions.

We'll need to set up a workflow that builds the project (basically a composer install). The workflow should do that, and:

• Test against a matrix of PHP 8.1, 8.2, and 8.3
• Also against Ubuntu, macOS, and Windows.
• The web directory should be served by Apache or nginx (we can't use the PHP built-in server because it's single-threaded).
• Run automatically every 12 hours, and when it's triggered manually.

[TravisCarden]

@phenaproxima, I can give you a PR that does all of this on Linux only that you can merge today. But macOS and Windows are taking a lot longer to figure out. Would you like an 80% value PR now and the rest in maybe a week? Or do you want to wait to get it all when I'm done?

[phenaproxima]

Let's take the 80%! Can you just open a follow-up issue to add support for Windows and macOS too?

[TravisCarden]

Okay, @phenaproxima; ^it's ready for you!