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/collab/src/tests/following_tests.rs

@@ -250,7 +250,7 @@ async fn test_basic_following(
     });
     executor.run_until_parked();
     // are you sure you want to leave the call?
-    cx_c.simulate_prompt_answer(0);
+    cx_c.simulate_prompt_answer("Close window and hang up");
     cx_c.cx.update(|_| {
         drop(workspace_c);
     });