Revert "refactor: use built-in wellformed operation (#774)"

This reverts commit 7f48cb6.

Author: mcollina

lib/serializer.js

@@ -1,5 +1,8 @@
 'use strict'
 
+// eslint-disable-next-line
+const STR_ESCAPE = /[\u0000-\u001f\u0022\u005c\ud800-\udfff]/
+
 module.exports = class Serializer {
   constructor (options) {
     switch (options && options.rounding) {
@@ -116,7 +119,7 @@ module.exports = class Serializer {
         }
       }
       return (last === -1 && ('"' + str + '"')) || ('"' + result + str.slice(last) + '"')
-    } else if (len < 5000 && str.isWellFormed()) {
+    } else if (len < 5000 && STR_ESCAPE.test(str) === false) {
       // Only use the regular expression for shorter input. The overhead is otherwise too much.
       return '"' + str + '"'
     } else {