Enhance `workspace::Open` to use as a more general directory explorer

Enhance workspace::Open to use as a more general directory explorer: As mentioned in #43961, the prompt_for_paths picker is already pretty close to providing functionality similar to helix's file explorer and similar interfaces. The following improvements would get it there: - Action to open it to the current directory rather than the worktree route - Alternate select action to accept the currently matched subdir (effectively narrowing your search to that subdir) without literally opening the directory in zed. - You can already navigate up a directory with ctrl-backspace to remove the previous path segment. If we think it's critical to the...