Update Prettier to version 2.0

Please note that these changes were done automatically, using `gulp lint --fix`.

Given that the major version number was increased, there's a fair number of (primarily whitespace) changes; please see https://prettier.io/blog/2020/03/21/2.0.0.html
In order to reduce the size of these changes somewhat, this patch maintains the old "arrowParens" style for now (once mozilla-central updates Prettier we can simply choose the same formatting, assuming it will differ here).

src/core/core_utils.js

@@ -18,7 +18,7 @@ import { assert, BaseException, warn } from "../shared/util.js";
 
 function getLookupTableFactory(initializer) {
   let lookup;
-  return function() {
+  return function () {
     if (initializer) {
       lookup = Object.create(null);
       initializer(lookup);