Replace prettier by stylistic to lint JavaScript (#3303)

In the latest versions of ESLint, more and more formatting rules were
removed or declared deprecated. These rules have been integrated into
the new Stylistic package (https://eslint.style/guide/why) and expanded.

Stylistic acts as a better formatter  for JavaScript as Prettier.

With this PR there are many changes that make the code more uniform, but
it may be difficult to review due to the large amount. Even if I have no
worries about the changes, perhaps this would be something for the
release after next.

Let me know what you think.

modules/default/newsfeed/node_helper.js

@@ -11,13 +11,13 @@ const NewsfeedFetcher = require("./newsfeedfetcher");
 
 module.exports = NodeHelper.create({
 	// Override start method.
-	start: function () {
+	start () {
 		Log.log(`Starting node helper for: ${this.name}`);
 		this.fetchers = [];
 	},
 
 	// Override socketNotificationReceived received.
-	socketNotificationReceived: function (notification, payload) {
+	socketNotificationReceived (notification, payload) {
 		if (notification === "ADD_FEED") {
 			this.createFetcher(payload.feed, payload.config);
 		}
@@ -29,7 +29,7 @@ module.exports = NodeHelper.create({
 	 * @param {object} feed The feed object
 	 * @param {object} config The configuration object
 	 */
-	createFetcher: function (feed, config) {
+	createFetcher (feed, config) {
 		const url = feed.url || "";
 		const encoding = feed.encoding || "UTF-8";
 		const reloadInterval = feed.reloadInterval || config.reloadInterval || 5 * 60 * 1000;
@@ -76,7 +76,7 @@ module.exports = NodeHelper.create({
 	 * Creates an object with all feed items of the different registered feeds,
 	 * and broadcasts these using sendSocketNotification.
 	 */
-	broadcastFeeds: function () {
+	broadcastFeeds () {
 		const feeds = {};
 		for (let f in this.fetchers) {
 			feeds[f] = this.fetchers[f].items();