A set of outline panel fixes (#12965)

Follow-up of https://github.com/zed-industries/zed/pull/12637

* Wrong font size for the outline items (fixes
https://github.com/zed-industries/zed/pull/12637#issuecomment-2164084021)
* Duplicate context menu item (fixes
https://github.com/zed-industries/zed/issues/12957)
* Missing `space` keybinding for item opening (fixes
https://github.com/zed-industries/zed/issues/12956)
* Adds 60px more to the default width (fixes
https://github.com/zed-industries/zed/issues/12955)
* Incorrect scroll for singleton buffers (fixes
https://github.com/zed-industries/zed/issues/12953)

Release Notes:

- N/A