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/platform.rs

@@ -45,6 +45,7 @@ use image::codecs::gif::GifDecoder;
 use image::{AnimationDecoder as _, Frame};
 use parking::Unparker;
 use raw_window_handle::{HasDisplayHandle, HasWindowHandle};
+use schemars::JsonSchema;
 use seahash::SeaHasher;
 use serde::{Deserialize, Serialize};
 use smallvec::SmallVec;
@@ -1244,7 +1245,7 @@ pub enum PromptLevel {
 }
 
 /// The style of the cursor (pointer)
-#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, Serialize, Deserialize, JsonSchema)]
 pub enum CursorStyle {
     /// The default cursor
     Arrow,