Add support for per-file LSP configuration

[itsmath]

Since it seems like luau-lang/rfcs#125 is generally supported and has a good likelihood of being merged, it would be useful if we could specify a different configuration for .config.luau files, in a Roblox project, this would prevent accidentally using the task library in config files.

This would also be useful for TestEZ, since you could include the definitions for it only in .spec.luau files. It could also be useful for Rojo projects, since .server.luau and .client.luau exist, but I can't think of specific use case for that at the moment.

I think the best way to do this is with glob patterns, but it could also be a simple file name suffix thing.

[itsmath]

This could also help with #1065, since you'd be able to set different configurations for files in src/Client, src/Shared and src/Server (potentially including filtering certain requires, but that is unrelated to this issue)

[itsmath]

Seems like this is sort of a duplicate of #29 (which I hadn't noticed), but this is more general. If possible at all, I'd love to be able to use pretty much all current configurations instead of just extra definitions. Let me know if I should close this in favor of #29.

[JohnnyMorganz]

This is harder than it sounds, because to change the loaded type definitions we need to load a new typechecker environment. At the moment, this is only done at the workspace level. Doing it at the file level is challenging, particularly if we want to be "extended" (i.e., subdirectories with configuration are an extension of configuration higher up the tree, as well as the default configuration for the language server)