Document how to setup dev environment

Issue Description

Quoting @carsakiller from https://github.com/LuaLS/LuaLS.github.io/pull/35#issuecomment-2134278430:

@sumneko would you be able to provide some more insight into how you develop the language server? I think I recall you saying that development is primarily test-driven. However, I'm not sure how the testing framework works — I believe it is custom? Some pointers on writing and running tests would be appreciated!

Is there a specific flow/method you would recommend for contributors? For the few simple things I have contributed, I have just used print() as described in this PR — but I don't think it would be very efficient for larger changes.

I think creating good documentation here for developing the language server would greatly help the community make some really nice pull requests. I've seen many comments from users eager to help but confused when getting their development environment set up and navigating the codebase.

Some initial work was recently merged in https://github.com/LuaLS/LuaLS.github.io/pull/35.

Additional Notes

No response

Prüfe die in LuaLS.github.io pull request 35 hinzugefügten Entwicklungshinweise und die verknüpfte Diskussion, um den angeforderten Kontext zu erhalten. Dokumentiere die Einrichtung der Umgebung, das Test-Framework und, wie man Tests schreibt und ausführt, sowie einen empfohlenen Workflow für Mitwirkende; abgeschlossen ist die Aufgabe, wenn ein Neuling dem Leitfaden folgen kann, ohne auf print()-Debugging angewiesen zu sein.