Add missing 'expansion' slot TypeScript definition

The DataTable.d.ts is missing the 'expansion' field
in the $slots class field.

src/components/datatable/DataTable.d.ts

@@ -114,6 +114,7 @@ declare class DataTable {
         groupheader: VNode[];
         groupfooter: VNode[];
         loading: VNode[];
+        expansion: VNode[];
     };
 }