Support private fields for classes with #-prefix

[ArneBab]

With this change the parser no longer treats private fields in Javascript classes (https://github.com/tc39/proposal-class-fields) as errors.

[ArneBab]

This is currently missing this test:

(js2-deftest-parse parse-class-private-field-no-init-assigned
  "class C {\n  #x\n  \"z\"\n  456\n  _setY() {  this.#x = 1;\n}\n}"
  :warnings-count 0)

[JeremyPouyet]

Works for me! Thanks. The possibility to customize the color would be a great addition.

[ArneBab]

@JeremyPouyet please have a look here for an option with less maintenance burden: #537 (comment)