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/text_system.rs

@@ -311,6 +311,10 @@ impl WindowTextSystem {
         self.line_layout_cache.reuse_layouts(index)
     }
 
+    pub(crate) fn truncate_layouts(&self, index: LineLayoutIndex) {
+        self.line_layout_cache.truncate_layouts(index)
+    }
+
     /// Shape the given line, at the given font_size, for painting to the screen.
     /// Subsets of the line can be styled independently with the `runs` parameter.
     ///