vim: Fix key navigation on folded buffer headers (#25944)

Closes #24243

Release Notes:

- vim: Fix j/k on folded multibuffer headers

---------

Co-authored-by: João Marcos <marcospb19@hotmail.com>

crates/editor/src/display_map.rs

@@ -1124,6 +1124,11 @@ impl DisplaySnapshot {
         self.block_snapshot.is_block_line(BlockRow(display_row.0))
     }
 
+    pub fn is_folded_buffer_header(&self, display_row: DisplayRow) -> bool {
+        self.block_snapshot
+            .is_folded_buffer_header(BlockRow(display_row.0))
+    }
+
     pub fn soft_wrap_indent(&self, display_row: DisplayRow) -> Option<u32> {
         let wrap_row = self
             .block_snapshot