Tests / CI / Github Actions

[joehoyle]

I think it would be great to set up some more CI checks and also start adding tests.

A great start would at least be PHP linting for all supported PHP versions, seems like a #good-first-issue. Is WP Env capable of doing that, or would using setup-php actions be the way to go?

[milesddavis]

Same for Wordpress versions. I doubt the plugin supports all versions back to 4.1 but it builds the packages for them.

[joehoyle]

Looks to be handled in #59