add .gitattributes and fix prettier/js warnings (#3094)

see title, as discussed in
https://github.com/MichMich/MagicMirror/pull/3093

tests/electron/modules/calendar_spec.js

@@ -3,7 +3,6 @@ const helpers = require("../helpers/global-setup");
 describe("Calendar module", () => {
 	/**
 	 * move similar tests in function doTest
-	 *
 	 * @param {string} cssClass css selector
 	 */
 	const doTest = async (cssClass) => {