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/agent/Cargo.toml

@@ -76,7 +76,6 @@ serde.workspace = true
 serde_json.workspace = true
 serde_json_lenient.workspace = true
 settings.workspace = true
-smallvec.workspace = true
 smol.workspace = true
 streaming_diff.workspace = true
 telemetry.workspace = true

crates/agent/src/message_editor.rs

@@ -842,7 +842,7 @@ impl MessageEditor {
             .border_b_0()
             .border_color(border_color)
             .rounded_t_md()
-            .shadow(smallvec::smallvec![gpui::BoxShadow {
+            .shadow(vec![gpui::BoxShadow {
                 color: gpui::black().opacity(0.15),
                 offset: point(px(1.), px(-1.)),
                 blur_radius: px(3.),