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/display/display_utils.js

@@ -502,7 +502,7 @@ function loadScript(src) {
     script.src = src;
 
     script.onload = resolve;
-    script.onerror = function() {
+    script.onerror = function () {
       reject(new Error(`Cannot load script at: ${script.src}`));
     };
     (document.head || document.documentElement).appendChild(script);