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/workspace/src/pane_group.rs

@@ -902,9 +902,9 @@ mod element {
     use std::{cell::RefCell, iter, rc::Rc, sync::Arc};
 
     use gpui::{
-        Along, AnyElement, App, Axis, BorderStyle, Bounds, Element, GlobalElementId, IntoElement,
-        MouseDownEvent, MouseMoveEvent, MouseUpEvent, ParentElement, Pixels, Point, Size, Style,
-        WeakEntity, Window, px, relative, size,
+        Along, AnyElement, App, Axis, BorderStyle, Bounds, Element, GlobalElementId,
+        HitboxBehavior, IntoElement, MouseDownEvent, MouseMoveEvent, MouseUpEvent, ParentElement,
+        Pixels, Point, Size, Style, WeakEntity, Window, px, relative, size,
     };
     use gpui::{CursorStyle, Hitbox};
     use parking_lot::Mutex;
@@ -1091,7 +1091,7 @@ mod element {
             };
 
             PaneAxisHandleLayout {
-                hitbox: window.insert_hitbox(handle_bounds, true),
+                hitbox: window.insert_hitbox(handle_bounds, HitboxBehavior::Normal),
                 divider_bounds,
             }
         }