Add/remove excerpts in BlockMap randomized tests and fix resulting errors

Co-Authored-By: Nathan Sobo <nathan@zed.dev>
Co-Authored-By: Max Brunsfeld <max@zed.dev>

crates/editor/src/display_map/wrap_map.rs

@@ -954,6 +954,10 @@ impl WrapPoint {
     pub fn column_mut(&mut self) -> &mut u32 {
         &mut self.0.column
     }
+
+    pub fn is_zero(&self) -> bool {
+        self.0.is_zero()
+    }
 }
 
 impl sum_tree::Summary for TransformSummary {