project search: Reduce hangs on main thread (#39857)

This takes the idea that @RemcoSmitsDev started on in
https://github.com/zed-industries/zed/pull/39354. We did away with
grabbing a snapshot of the display map when buffer coordinates were
sufficient.
Closes #37267

Release Notes:

- Reduced micro-stutters in project search with large multi-buffer
contents.

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>

crates/multi_buffer/src/multi_buffer.rs

@@ -6385,6 +6385,17 @@ impl MultiBufferSnapshot {
             debug_ranges.insert(key, text_ranges, format!("{value:?}").into())
         });
     }
+
+    // used by line_mode selections and tries to match vim behavior
+    pub fn expand_to_line(&self, range: Range<Point>) -> Range<Point> {
+        let new_start = MultiBufferPoint::new(range.start.row, 0);
+        let new_end = if range.end.column > 0 {
+            MultiBufferPoint::new(range.end.row, self.line_len(MultiBufferRow(range.end.row)))
+        } else {
+            range.end
+        };
+        new_start..new_end
+    }
 }
 
 #[cfg(any(test, feature = "test-support"))]