Investigate performance of workspace diagnostics when disabled

[JohnnyMorganz]

Similar to #749, profiling shows that workspace diagnostics take a significant amount of time (30s). Although, much of it might be due to the start up cost of type checking the whole workspace, however profiling does not show us any significant calls to Frontend::check:

Also, findFilesForWorkspaceDiagnostics takes 2s, which can probably be improved further to prevent delays whilst workspace diagnostics are being computed on a fully cached repository. (workspace diagnostics should be instanteneous if there have been no changes to files)

[JohnnyMorganz]

Actually, this is with luau-lsp.diagnostics.workspace disabled, which is pretty bad