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/language_tools/src/syntax_tree_view.rs

@@ -365,7 +365,7 @@ impl Render for SyntaxTreeView {
             rendered = rendered.child(
                 canvas(
                     move |bounds, cx| {
-                        list.layout(bounds.origin, bounds.size.into(), cx);
+                        list.prepaint_as_root(bounds.origin, bounds.size.into(), cx);
                         list
                     },
                     |_, mut list, cx| list.paint(cx),