fix: add missing externs

externs.js

@@ -34,6 +34,7 @@ dummy.folder = function() {};
 dummy.file = function() {};
 dummy.generateAsync = function() {};
 dummy.showOpenFilePicker = function() {};
+dummy.showDirectoryPicker = function() {};
 dummy.getDirectoryHandle = function() {};
 dummy.removeEntry = function() {};
 dummy.getFile = function() {};