Make versions oldest_selection and newest_selection that don't require snapshots

I thought I needed this but actually didn't, but I still kinda think it's a good change for the public interface of Editor.

crates/go_to_line/src/go_to_line.rs

@@ -54,7 +54,7 @@ impl GoToLine {
             let buffer = editor.buffer().read(cx).read(cx);
             (
                 Some(scroll_position),
-                editor.newest_selection(&buffer).head(),
+                editor.newest_selection_with_snapshot(&buffer).head(),
                 buffer.max_point(),
             )
         });