Add indexer support for mixins

[pgenfer]

Indexers in mixins should also be included in the child class and forwarded to the mixin instance.

public class Mixin
{
       public int this[int index]{/* indexer logic */}
}

should be forwarded to

public class Child
{
       private Mixin _mixin;
       public int this[int index] => _mixin[index];
}