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/vim/src/command.rs

@@ -1695,12 +1695,10 @@ mod test {
         // conflict!
         cx.simulate_keystrokes("i @ escape");
         cx.simulate_keystrokes(": w enter");
-        assert!(cx.has_pending_prompt());
-        // "Cancel"
-        cx.simulate_prompt_answer(0);
+        cx.simulate_prompt_answer("Cancel");
+
         assert_eq!(fs.load(path).await.unwrap().replace("\r\n", "\n"), "oops\n");
         assert!(!cx.has_pending_prompt());
-        // force overwrite
         cx.simulate_keystrokes(": w ! enter");
         assert!(!cx.has_pending_prompt());
         assert_eq!(fs.load(path).await.unwrap().replace("\r\n", "\n"), "@@\n");