Scroll to follow expanding part of editor::SelectLargerSyntaxNode (#27295)

When the selection grows both ways, the new code prioritizes the top
part instead of bottom one, this is usually more helpful considering
that most programming language grammars tend to define tokens right
before large delimited blocks, and rarely after (because humans and
parsers read from top to bottom).

Also, revert selection when convenient, so you have more control over
what you're selecting, looking at the selection `head` is commonly more
convenient than at the `tail`.

Release Notes:

- Improve scrolling of `editor::SelectLargerSyntaxNode` for better
visibility.

crates/editor/src/scroll/autoscroll.rs

@@ -38,6 +38,16 @@ impl Autoscroll {
         Self::Strategy(AutoscrollStrategy::TopRelative(n))
     }
 
+    /// Scrolls so that the newest cursor is at the top.
+    pub fn top() -> Self {
+        Self::Strategy(AutoscrollStrategy::Top)
+    }
+
+    /// Scrolls so that the newest cursor is roughly an n-th line from the bottom.
+    pub fn bottom_relative(n: usize) -> Self {
+        Self::Strategy(AutoscrollStrategy::BottomRelative(n))
+    }
+
     /// Scrolls so that the newest cursor is at the bottom.
     pub fn bottom() -> Self {
         Self::Strategy(AutoscrollStrategy::Bottom)
@@ -54,6 +64,7 @@ pub enum AutoscrollStrategy {
     Top,
     Bottom,
     TopRelative(usize),
+    BottomRelative(usize),
 }
 
 impl AutoscrollStrategy {
@@ -213,6 +224,10 @@ impl Editor {
                 scroll_position.y = target_top - lines as f32;
                 self.set_scroll_position_internal(scroll_position, local, true, window, cx);
             }
+            AutoscrollStrategy::BottomRelative(lines) => {
+                scroll_position.y = target_bottom + lines as f32;
+                self.set_scroll_position_internal(scroll_position, local, true, window, cx);
+            }
         }
 
         self.scroll_manager.last_autoscroll = Some((