[CI]: Integration tests for various OSes, busted for testing

[vhyrro]

Since we're now invested in luarocks we might as well start using proper libraries for testing the entirety of Neorg.

I would like to start taking CI runs more seriously, including: having CI automatically set up Neorg on various operating systems to see if it works out of the box, running tests on individual modules to ensure they work as expected, blocking merging of PRs until things like type checking passes.

I've already gained a lot of valuable experience from working on rocks.nvim, all of that can be translated over to Neorg in no time!

[vhyrro]

CI is up and running (with the exclusion of windows, trying to get that set up). Busted tests are next up on the list afterwards!

[vhyrro]

Everything is fully up and running and it works flawlessly! Next up: making luarocks-tag-release run busted tests. After that we can close this issue :)

[vhyrro]

All the logic is set up now! Next up: actually write the tests for the codebase