Only allow save_as for pane if can_save_as is true (#25028)

When saving an item, some logic is done to determine whether one can
save it. In the special case where the intent is to `SaveAs`, it was
previously allowed to proceed as long as the buffer was a singleton
(presumably since it only makes sense to provide a save path for a
single file). However, we need to _also_ check that this item can be
"saved as" at all.

For this, we resurrect the `ItemHandle`/`Item` trait method
`can_save_as`. We have given it the default implementation of returning
`false`, and then overridden this in the implementation for
`TerminalView`.

Closes #25023


Release Notes:

- Fixed crash when trying to save terminal buffer

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/editor/src/items.rs

@@ -706,6 +706,10 @@ impl Item for Editor {
         self.buffer.read(cx).is_singleton()
     }
 
+    fn can_save_as(&self, cx: &App) -> bool {
+        self.buffer.read(cx).is_singleton()
+    }
+
     fn clone_on_split(
         &self,
         _workspace_id: Option<WorkspaceId>,