JS: Add metadata to dbscheme and stats

javascript/ql/lib/semmlecode.javascript.dbscheme

@@ -1192,3 +1192,9 @@ configLocations(
 );
 
 @configLocatable = @config | @configName | @configValue;
+
+/*- Database metadata -*/
+databaseMetadata(
+  string metadataKey: string ref,
+  string value: string ref
+);