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/assistant_tools/src/edit_agent/evals/fixtures/disable_cursor_blinking/before.rs

@@ -7698,7 +7698,7 @@ impl Editor {
                     .gap_1()
                     // Workaround: For some reason, there's a gap if we don't do this
                     .ml(-BORDER_WIDTH)
-                    .shadow(smallvec![gpui::BoxShadow {
+                    .shadow(vec![gpui::BoxShadow {
                         color: gpui::black().opacity(0.05),
                         offset: point(px(1.), px(1.)),
                         blur_radius: px(2.),