project_panel: Add file comparison function, supports selecting files for comparison (#35255)

Closes https://github.com/zed-industries/zed/discussions/35010
Closes https://github.com/zed-industries/zed/issues/17100
Closes https://github.com/zed-industries/zed/issues/4523

Release Notes:

- Added file comparison function in project panel

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Co-authored-by: Kirill Bulatov <kirill@zed.dev>

assets/keymaps/default-linux.json

@@ -848,6 +848,7 @@
       "ctrl-delete": ["project_panel::Delete", { "skip_prompt": false }],
       "alt-ctrl-r": "project_panel::RevealInFileManager",
       "ctrl-shift-enter": "project_panel::OpenWithSystem",
+      "alt-d": "project_panel::CompareMarkedFiles",
       "shift-find": "project_panel::NewSearchInDirectory",
       "ctrl-alt-shift-f": "project_panel::NewSearchInDirectory",
       "shift-down": "menu::SelectNext",