Set up stylua and selene.

[aatxe]

Some day, we'll want luau-native versions of these tools that we can dogfood with, but for now, it'd be nice to have stylua and selene setup to help folks contribute good quality luau to the project. Plenty of folks have already set this up themselves in other environments, so I imagine this is a pretty accessible one setup-wise. I'd suggest we probably use rokit if we're going to do a toolchain manager for it as well.