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/editor/src/editor.rs

@@ -21907,7 +21907,7 @@ fn render_diff_hunk_controls(
         .rounded_b_lg()
         .bg(cx.theme().colors().editor_background)
         .gap_1()
-        .stop_mouse_events_except_scroll()
+        .block_mouse_except_scroll()
         .shadow_md()
         .child(if status.has_secondary_hunk() {
             Button::new(("stage", row as u64), "Stage")