Add "Safe" (nullable) cast operator as?

[wrozwad]

It'll be a little enhancement but so much pleasant one :)

Kotlin and Swift has something like Safe casting with as? operator. It gives an opportunity to casting with null fallback if cast can't be performed:

void someFunction(dynamic arg) {
  // we could has something like
  final classVariable = (arg as? SomeClass)?.someValue;
  
  // in place of
  final classVariable = arg is SomeClass ? arg.someValue : null;
}

[wrozwad]

Duplicate of #399