Add initial element inspector for Zed development (#31315)

Open inspector with `dev: toggle inspector` from command palette or
`cmd-alt-i` on mac or `ctrl-alt-i` on linux.

https://github.com/user-attachments/assets/54c43034-d40b-414e-ba9b-190bed2e6d2f

* Picking of elements via the mouse, with scroll wheel to inspect
occluded elements.

* Temporary manipulation of the selected element.

* Layout info and JSON-based style manipulation for `Div`.

* Navigation to code that constructed the element.

Big thanks to @as-cii and @maxdeviant for sorting out how to implement
the core of an inspector.

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Marshall Bowers <git@maxdeviant.com>
Co-authored-by: Federico Dionisi <code@fdionisi.me>

assets/keymaps/default-linux.json

@@ -675,7 +675,7 @@
   {
     "bindings": {
       "ctrl-alt-shift-f": "workspace::FollowNextCollaborator",
-      "ctrl-alt-i": "zed::DebugElements"
+      "ctrl-alt-i": "dev::ToggleInspector"
     }
   },
   {

assets/keymaps/default-macos.json

@@ -735,7 +735,7 @@
       "ctrl-alt-cmd-f": "workspace::FollowNextCollaborator",
       // TODO: Move this to a dock open action
       "cmd-shift-c": "collab_panel::ToggleFocus",
-      "cmd-alt-i": "zed::DebugElements"
+      "cmd-alt-i": "dev::ToggleInspector"
     }
   },
   {