Add/Finalize/Document support for Lua Mods

Everest has some support for mods written in Lua ([Everest.LuaLoader.cs](https://github.com/EverestAPI/Everest/blob/dev/Celeste.Mod.mm/Mod/Everest/Everest.LuaLoader.cs)), notably used by the [Lua Cutscenes](https://github.com/Cruor/LuaCutscenes) Mod, but there is no documentation on how to use it, and it is unknown how much of it currently works.