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/pattern.js

@@ -51,7 +51,7 @@ var Pattern = (function PatternClosure() {
     },
   };
 
-  Pattern.parseShading = function(
+  Pattern.parseShading = function (
     shading,
     matrix,
     xref,