[NotHyper-474]

This PR allows enum arguments to be optional using the ?name:Type syntax, also allowing switch case enum matching to specify null as a value as a quick way to check for an argument that was passed null (or not passed at all).

enum Foo
{
  Aab(yes:String, ?no:String);
}

var foo = Foo.Aab('ok');
switch (foo)
{
  case Aab(yes, null):
    trace('Got Aab with Yes: "$yes"');
  case Aab(yes): // Directly equivalent to the one above
  case Aab(yes, no): // Doesn't run because the order matters (that's the behavior on actual Haxe: https://try.haxe.org/#7dbb3855)
    trace('Got Aab with Yes: "$yes" and No: "$no"');
  default:
}