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>

crates/zeta/src/completion_diff_element.rs

@@ -105,9 +105,14 @@ impl Element for CompletionDiffElement {
         None
     }
 
+    fn source_location(&self) -> Option<&'static core::panic::Location<'static>> {
+        None
+    }
+
     fn request_layout(
         &mut self,
         _id: Option<&gpui::GlobalElementId>,
+        _inspector_id: Option<&gpui::InspectorElementId>,
         window: &mut Window,
         cx: &mut App,
     ) -> (gpui::LayoutId, Self::RequestLayoutState) {
@@ -117,6 +122,7 @@ impl Element for CompletionDiffElement {
     fn prepaint(
         &mut self,
         _id: Option<&gpui::GlobalElementId>,
+        _inspector_id: Option<&gpui::InspectorElementId>,
         _bounds: gpui::Bounds<Pixels>,
         _request_layout: &mut Self::RequestLayoutState,
         window: &mut Window,
@@ -128,6 +134,7 @@ impl Element for CompletionDiffElement {
     fn paint(
         &mut self,
         _id: Option<&gpui::GlobalElementId>,
+        _inspector_id: Option<&gpui::InspectorElementId>,
         _bounds: gpui::Bounds<Pixels>,
         _request_layout: &mut Self::RequestLayoutState,
         _prepaint: &mut Self::PrepaintState,