Require save confirmation and prevent autosave for deleted files (#20742)

* `has_conflict` will now return true if the file has been deleted on
disk.  This is for treating multi-buffers as conflicted, and also
blocks auto-save.

* `has_deleted_file` is added so that the single-file buffer save can
specifically mention the delete conflict. This does not yet handle
discard (#20745).

Closes #9101
Closes #9568
Closes #20462

Release Notes:

- Improved handling of externally deleted files: auto-save will be
disabled, multibuffers will treat this as a save conflict, and single
buffers will ask for restore confirmation.

Co-authored-by: Conrad <conrad@zed.dev>