member types, e.g. Tree<T>.Node

[d180cf]

Sometimes it's useful to declare new types within a class/interface, especially when it's a generic:

class Tree<T> {
  private type Node = { data: T, subnodes: Node[] };
  public root: Node = { data: null, subnodes: [] };
  ...
}

Such member types could be referred to as Tree<T>.Node.

[Arnavion]

Previous discussion at #7061

[mhegazy]

Duplicate of #7061 as noted by @Arnavion