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/markdown/src/markdown.rs

@@ -3,6 +3,7 @@ mod path_range;
 
 use base64::Engine as _;
 use futures::FutureExt as _;
+use gpui::HitboxBehavior;
 use language::LanguageName;
 use log::Level;
 pub use path_range::{LineCol, PathWithRange};
@@ -1211,7 +1212,7 @@ impl Element for MarkdownElement {
         window.set_focus_handle(&focus_handle, cx);
         window.set_view_id(self.markdown.entity_id());
 
-        let hitbox = window.insert_hitbox(bounds, false);
+        let hitbox = window.insert_hitbox(bounds, HitboxBehavior::Normal);
         rendered_markdown.element.prepaint(window, cx);
         self.autoscroll(&rendered_markdown.text, window, cx);
         hitbox