workspace: Fix read-only pane buttons being clickable even when a no-op (#46065)

The button was clickable for read-only panes even if the underlying item
is not write-toggleable.

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/multi_buffer/src/multi_buffer.rs

@@ -1198,6 +1198,10 @@ impl MultiBuffer {
         !self.capability.editable()
     }
 
+    pub fn capability(&self) -> Capability {
+        self.capability
+    }
+
     /// Returns an up-to-date snapshot of the MultiBuffer.
     #[ztracing::instrument(skip_all)]
     pub fn snapshot(&self, cx: &App) -> MultiBufferSnapshot {