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/line_layout.rs

@@ -347,6 +347,14 @@ impl LineLayoutCache {
         }
     }
 
+    pub fn truncate_layouts(&self, index: LineLayoutIndex) {
+        let mut current_frame = &mut *self.current_frame.write();
+        current_frame.used_lines.truncate(index.lines_index);
+        current_frame
+            .used_wrapped_lines
+            .truncate(index.wrapped_lines_index);
+    }
+
     pub fn finish_frame(&self) {
         let mut prev_frame = self.previous_frame.lock();
         let mut curr_frame = self.current_frame.write();