feature: Create `UnindexedProject` notification to be sent to the client
(Note that this branch contains commits from https://github.com/rust-lang/rust-analyzer/pull/15830, which I'll rebase atop of as needed.)
Based on the discussion in https://github.com/rust-lang/rust-analyzer/issues/15837, I've added a notification and off-by-default toggle to send that notification from `handle_did_open_text_document`. I'm happy to rename/tweak this as needed.
I've been using this for a little bit, and it does seem to cause a little bit more indexing/work in rust-analyzer, but it's something that I'll profile as needed, I think.