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/ui/src/components/button/split_button.rs

@@ -41,7 +41,7 @@ impl RenderOnce for SplitButton {
             )
             .child(self.right)
             .bg(ElevationIndex::Surface.on_elevation_bg(cx))
-            .shadow(smallvec::smallvec![BoxShadow {
+            .shadow(vec![BoxShadow {
                 color: hsla(0.0, 0.0, 0.0, 0.16),
                 offset: point(px(0.), px(1.)),
                 blur_radius: px(0.),