Add commands to select next/previous siblings in the syntax tree (#35053)

Closes #5133 and discussion
https://github.com/zed-industries/zed/discussions/33493

This PR adds two new commands to select next/previous siblings in the
syntax tree. These commands were modelled after the existing ones about
expand/shrink selection. With this PR I've added new key bindings
inspired by `helix` for previous / next / expand / shrink selections.



https://github.com/user-attachments/assets/4ef7fadb-0b82-4897-95c7-1737827bf4ac


Release Notes:

- Add commands to select next/previous siblings in the syntax tree

---------

Co-authored-by: Joseph T. Lyons <JosephTLyons@gmail.com>

crates/multi_buffer/src/multi_buffer.rs

@@ -6095,6 +6095,28 @@ impl MultiBufferSnapshot {
         Some((node, range))
     }
 
+    pub fn syntax_next_sibling<T: ToOffset>(
+        &self,
+        range: Range<T>,
+    ) -> Option<tree_sitter::Node<'_>> {
+        let range = range.start.to_offset(self)..range.end.to_offset(self);
+        let mut excerpt = self.excerpt_containing(range.clone())?;
+        excerpt
+            .buffer()
+            .syntax_next_sibling(excerpt.map_range_to_buffer(range))
+    }
+
+    pub fn syntax_prev_sibling<T: ToOffset>(
+        &self,
+        range: Range<T>,
+    ) -> Option<tree_sitter::Node<'_>> {
+        let range = range.start.to_offset(self)..range.end.to_offset(self);
+        let mut excerpt = self.excerpt_containing(range.clone())?;
+        excerpt
+            .buffer()
+            .syntax_prev_sibling(excerpt.map_range_to_buffer(range))
+    }
+
     pub fn outline(&self, theme: Option<&SyntaxTheme>) -> Option<Outline<Anchor>> {
         let (excerpt_id, _, buffer) = self.as_singleton()?;
         let outline = buffer.outline(theme)?;