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/assistant/src/assistant_panel.rs

@@ -1108,7 +1108,7 @@ impl AssistantPanel {
                             )
                             .track_scroll(scroll_handle)
                             .into_any_element();
-                            saved_conversations.layout(
+                            saved_conversations.prepaint_as_root(
                                 bounds.origin,
                                 bounds.size.map(AvailableSpace::Definite),
                                 cx,