The "Expand All Files"/"Collapse All Files" button in `BufferSearchBar`
was broken for `SplittableEditor`, which is used in the project diff
view. It was happening because `ProjectDiff::as_searchable` returns an
handle to the `SplittableEditor`, which the search bar implementation
then tries to downcast to an `Editor`, which the `SplittableEditor` did
not support, so both the expand/collapse all buttons, as well as the
collapse state were broken.
Unfortunately this was accidentally introduced in
https://github.com/zed-industries/zed/pull/48773 , so this Pull Request
updates the `Item` implementation for `SplittableEditor` in order for it
to be able to act as an `Editor`.
Release Notes:
- Fix the "Expand All Files"/"Collapse All Files" button in the project
diff view
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Co-authored-by: Tom Houlé <tom@tomhoule.com>
