Fix case-only renaming of files in project panel (#7835)

This is a follow-up to #7768 but now also fixes #5211.

Explanation is relatively simple: case-only renames previously failed
because while Zed would think that `foobar` and `FOOBAR` are different,
the filesystem would give us an file-already-exists error when renaming.

So what we're doing here is to check whether we're on a case-insensitive
filesystem and if so, we overwrite the old file.

Release Notes:

- Fixed case-only renaming of files in project panel.
([#5211](https://github.com/zed-industries/zed/issues/5211)).

Proof:



https://github.com/zed-industries/zed/assets/1185253/57d5063f-09d9-47b1-a2df-3d7edefca97d