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/gpui/src/app/test_context.rs

@@ -734,7 +734,8 @@ impl VisualTestContext {
         self.update(|cx| {
             cx.with_element_context(|cx| {
                 let mut element = f(cx);
-                element.layout(origin, space, cx);
+                element.layout_as_root(space, cx);
+                cx.with_absolute_element_offset(origin, |cx| element.prepaint(cx));
                 element.paint(cx);
             });