[javascript] Support static class fields

[ris58h]

Currently javascript grammar doesn't support static fields in classes.

Input:

class S {
  static s
}

Errors:

line 3:0 no viable alternative at input 'class S {\n  static s\n}'
line 1:8 no viable alternative at input '{'
line 2:9 no viable alternative at input 's'

Specification: https://tc39.es/ecma262/multipage/ecmascript-language-functions-and-classes.html#prod-ClassElement