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/worker_stream.js

@@ -42,7 +42,7 @@ class PDFWorkerStream {
       this._fullRequestReader.cancel(reason);
     }
     const readers = this._rangeRequestReaders.slice(0);
-    readers.forEach(function(reader) {
+    readers.forEach(function (reader) {
       reader.cancel(reason);
     });
   }