gpui: Support hitbox blocking mouse interaction except scrolling (#31712)

tl;dr: This adds `.block_mouse_except_scroll()` which should typically
be used instead of `.occlude()` for cases when the mouse shouldn't
interact with elements drawn below an element. The rationale for
treating scroll events differently:

* Mouse move / click / styles / tooltips are for elements the user is
interacting with directly.
* Mouse scroll events are about finding the current outer scroll
container.

Most use of `occlude` should probably be switched to this, but I figured
I'd derisk this change by minimizing behavior changes to just the 3 uses
of `block_mouse_except_scroll`.

GPUI changes:

* Added `InteractiveElement::block_mouse_except_scroll()`, and removes
`stop_mouse_events_except_scroll()`

* Added `Hitbox::should_handle_scroll()` to be used when handling scroll
wheel events.

* `Window::insert_hitbox` now takes `HitboxBehavior` instead of
`occlude: bool`.

    - `false` for that bool is now `HitboxBehavior::Normal`.

    - `true` for that bool is now `HitboxBehavior::BlockMouse`.
    
    - The new mode is `HitboxBehavior::BlockMouseExceptScroll`.

* Removes `Default` impl for `HitboxId` since applications should not
manually create `HitboxId(0)`.

Release Notes:

- N/A

crates/gpui/examples/window_shadow.rs

@@ -1,8 +1,8 @@
 use gpui::{
-    App, Application, Bounds, Context, CursorStyle, Decorations, Hsla, MouseButton, Pixels, Point,
-    ResizeEdge, Size, Window, WindowBackgroundAppearance, WindowBounds, WindowDecorations,
-    WindowOptions, black, canvas, div, green, point, prelude::*, px, rgb, size, transparent_black,
-    white,
+    App, Application, Bounds, Context, CursorStyle, Decorations, HitboxBehavior, Hsla, MouseButton,
+    Pixels, Point, ResizeEdge, Size, Window, WindowBackgroundAppearance, WindowBounds,
+    WindowDecorations, WindowOptions, black, canvas, div, green, point, prelude::*, px, rgb, size,
+    transparent_black, white,
 };
 
 struct WindowShadow {}
@@ -37,7 +37,7 @@ impl Render for WindowShadow {
                                         point(px(0.0), px(0.0)),
                                         window.window_bounds().get_bounds().size,
                                     ),
-                                    false,
+                                    HitboxBehavior::Normal,
                                 )
                             },
                             move |_bounds, hitbox, window, _cx| {