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/gpui/src/gpui.rs

@@ -81,6 +81,7 @@ mod executor;
 mod geometry;
 mod global;
 mod input;
+mod inspector;
 mod interactive;
 mod key_dispatch;
 mod keymap;
@@ -135,6 +136,7 @@ pub use global::*;
 pub use gpui_macros::{AppContext, IntoElement, Render, VisualContext, register_action, test};
 pub use http_client;
 pub use input::*;
+pub use inspector::*;
 pub use interactive::*;
 use key_dispatch::*;
 pub use keymap::*;