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/jpx_stream.js

@@ -42,12 +42,12 @@ const JpxStream = (function JpxStreamClosure() {
     configurable: true,
   });
 
-  JpxStream.prototype.ensureBuffer = function(requested) {
+  JpxStream.prototype.ensureBuffer = function (requested) {
     // No-op, since `this.readBlock` will always parse the entire image and
     // directly insert all of its data into `this.buffer`.
   };
 
-  JpxStream.prototype.readBlock = function() {
+  JpxStream.prototype.readBlock = function () {
     if (this.eof) {
       return;
     }