file finder: Fix `./` breaking new-path prompt (#15438)

Fixes #15426.

The `./` was implicitly assumed to be there by the prompt, so we'd end
up with `././foobar` when typing in an explicit `./`.

This fixes the issue by stripping `./` from the query, like we also
strip `/`.

Release Notes:

- Fixed paths starting with `./` breaking the new-path file picker when
the system prompts are disabled.
([#15426](https://github.com/zed-industries/zed/issues/15426)).