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/image_utils.js

@@ -62,7 +62,7 @@ class NativeImageDecoder {
         image.getIR(this.forceDataSchema),
         colorSpace.numComps,
       ])
-      .then(function({ data, width, height }) {
+      .then(function ({ data, width, height }) {
         return new Stream(data, 0, data.length, dict);
       });
   }