Meta: Add basic GitHub Actions workflow for ubuntu/macOS builds

[ADKaster]

Let's verify that things at least build on pushes to master, and in PRs.

A few succesful CI runs from my fork showing the ccache working:

No ccache: https://github.com/ADKaster/ladybird/actions/runs/3075281272

Yes ccache: https://github.com/ADKaster/ladybird/actions/runs/3075390659

Creating UCD/CLDR/TZDB caches: https://github.com/ADKaster/ladybird/actions/runs/3078277536/jobs/4973731841

Full UCD/CLDR/TZDB caches: https://github.com/ADKaster/ladybird/actions/runs/3078478190

[trflynn89]

You might want to add the UCD/CLDR/TZDB caches as well so we aren't hitting their servers as much. At least the UCD, since they host that on a server that isn't behind a CDN.

[themisir]

What about publishing build artifacts?

https://github.com/actions/upload-artifact

[ADKaster]

Publishing build artifacts is a non-goal. It's very easy to build the project.

[themisir]

Publishing build artifacts is a non-goal. It's very easy to build the project.

Makes sense, thanks