[ty] Refactor how workspace folder initialization is tracked
Previously, we only kept a count of the number of workspaces
initialized, but we had no way of knowing *which* workspaces were
initialized. Knowing which are initialized is useful for avoiding
initializing a workspace again.
This is relevant because previously we only called
`initialize_workspace_folder` at actual initialization time. But soon
we're going to want to call it at arbitrary points in response to
`workspace/didChangeWorkspaceFolders` notifications. Making
initialization a no-op if a workspace is already initialized seems more
flexible and harder to misuse. With that said, presumably, LSP clients
won't try to re-initialize workspace folders.
This does change a couple checks that were previously constant time to
be linear over the number of workspace folders. I think this is fine
given that the unit of work here is tiny and the number of workspace
folders is not expected to be enormous. (The point at which a linear
scan over all workspaces is problematic likely means we have much bigger
problems.)
