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>

Cargo.toml

@@ -73,6 +73,7 @@ members = [
     "crates/indexed_docs",
     "crates/inline_completion",
     "crates/inline_completion_button",
+    "crates/inspector_ui",
     "crates/install_cli",
     "crates/jj",
     "crates/jj_ui",
@@ -279,6 +280,7 @@ image_viewer = { path = "crates/image_viewer" }
 indexed_docs = { path = "crates/indexed_docs" }
 inline_completion = { path = "crates/inline_completion" }
 inline_completion_button = { path = "crates/inline_completion_button" }
+inspector_ui = { path = "crates/inspector_ui" }
 install_cli = { path = "crates/install_cli" }
 jj = { path = "crates/jj" }
 jj_ui = { path = "crates/jj_ui" }
@@ -447,6 +449,7 @@ futures-batch = "0.6.1"
 futures-lite = "1.13"
 git2 = { version = "0.20.1", default-features = false }
 globset = "0.4"
+hashbrown = "0.15.3"
 handlebars = "4.3"
 heck = "0.5"
 heed = { version = "0.21.0", features = ["read-txn-no-tls"] }