vim: Fix :wq in multibuffer (#24603)

Supercedes #24561
Closes #21059

Before this change we would skip saving multibuffers regardless of the
save intent. Now we correctly save them.

Along the way:
* Prompt to save when closing the last singleton copy of an item (even
if it's still open in a multibuffer).
* Update our file name prompt to pull out dirty project items from
multibuffers instead of counting multibuffers as untitled files.
* Fix our prompt test helpers to require passing the button name instead
of the index. A few tests were passing invalid responses to save
prompts.
* Refactor the code a bit to hopefully clarify it for the next bug.

Release Notes:

- Fixed edge-cases when closing multiple items including multibuffers.
Previously no prompt was generated when closing an item that was open in
a multibuffer, now you will be prompted.
- vim: Fix :wq in a multibuffer

crates/recent_projects/src/recent_projects.rs

@@ -694,8 +694,7 @@ mod tests {
             cx.has_pending_prompt(),
             "Dirty workspace should prompt before opening the new recent project"
         );
-        // Cancel
-        cx.simulate_prompt_answer(0);
+        cx.simulate_prompt_answer("Cancel");
         assert!(
             !cx.has_pending_prompt(),
             "Should have no pending prompt after cancelling"