language: Only block the foreground on buffer reparsing when necessary (#43888)

Gist is we only need to block the foreground thread for reparsing if
immediate language changes are useful to the user. That is usually only
the case when they edit the buffer

Release Notes:

- Improved performance of large project searches and project diffs

Co-authored by: David Kleingeld <david@zed.dev>

crates/sum_tree/src/cursor.rs

@@ -403,6 +403,10 @@ where
     }
 
     /// Returns whether we found the item you were seeking for.
+    ///
+    /// # Panics
+    ///
+    /// If we did not seek before, use seek instead in that case.
     #[track_caller]
     pub fn seek_forward<Target>(&mut self, pos: &Target, bias: Bias) -> bool
     where