Introduce autoscroll support for elements (#10889)

This pull request introduces the new
`ElementContext::request_autoscroll(bounds)` and
`ElementContext::take_autoscroll()` methods in GPUI. These new APIs
enable container elements such as `List` to change their scroll position
if one of their children requested an autoscroll. We plan to use this in
the revamped assistant.

As a drive-by, we also:

- Renamed `Element::before_layout` to `Element::request_layout`
- Renamed `Element::after_layout` to `Element::prepaint`
- Introduced a new `List::splice_focusable` method to splice focusable
elements into the list, which enables rendering offscreen elements that
are focused.

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/editor/src/editor.rs

@@ -10801,7 +10801,7 @@ pub fn diagnostic_block_renderer(diagnostic: Diagnostic, _is_valid: bool) -> Ren
 
         let icon_size = buttons(&diagnostic, cx.block_id)
             .into_any_element()
-            .measure(AvailableSpace::min_size(), cx);
+            .layout_as_root(AvailableSpace::min_size(), cx);
 
         h_flex()
             .id(cx.block_id)