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/editor/src/editor.rs

@@ -138,7 +138,6 @@ pub use git::blame::BlameRenderer;
 pub use proposed_changes_editor::{
     ProposedChangeLocation, ProposedChangesEditor, ProposedChangesEditorToolbar,
 };
-use smallvec::smallvec;
 use std::{cell::OnceCell, iter::Peekable, ops::Not};
 use task::{ResolvedTask, RunnableTag, TaskTemplate, TaskVariables};
 
@@ -176,7 +175,7 @@ use selections_collection::{
 };
 use serde::{Deserialize, Serialize};
 use settings::{Settings, SettingsLocation, SettingsStore, update_settings_file};
-use smallvec::SmallVec;
+use smallvec::{SmallVec, smallvec};
 use snippet::Snippet;
 use std::sync::Arc;
 use std::{
@@ -7993,7 +7992,7 @@ 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.),
@@ -16708,7 +16707,7 @@ impl Editor {
     }
 
     pub fn wrap_guides(&self, cx: &App) -> SmallVec<[(usize, bool); 2]> {
-        let mut wrap_guides = smallvec::smallvec![];
+        let mut wrap_guides = smallvec![];
 
         if self.show_wrap_guides == Some(false) {
             return wrap_guides;