Fix scrollbar markers for folded code (#11625)

There're two errors in scrollbar markers in the presence of folded code:

1. Some markers are not displayed (when the marked row numbers are
greater than the total displayed rows count after folding).
2. Code folding / unfolding doesn't trigger markers repainting.

This PR fixes both problems.

Release Notes:

- Fixed scrollbar markers for folded code.

The second problem (markers are repainted after I move the cursor, not
after folding):


https://github.com/zed-industries/zed/assets/2101250/57ed563d-186d-4497-98ab-d4f946416726

crates/editor/src/editor.rs

@@ -9225,6 +9225,8 @@ impl Editor {
                     self.active_diagnostics = Some(active_diagnostics);
                 }
             }
+
+            self.scrollbar_marker_state.dirty = true;
         }
     }
 
@@ -9258,6 +9260,7 @@ impl Editor {
             }
 
             cx.notify();
+            self.scrollbar_marker_state.dirty = true;
         }
     }