vim: Fix :wq in multibuffer

[ConradIrwin]

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