Use "types" in frankdoc for flow child element validation

The types property in the frankdoc json contains key value structure where the key is the interface classname and the value is an array of frank element classnames.
This interface can be found when an element has children of a specific interface type (for example senderpipe has senders).

I can make a utility function in the doc-library so you can easily get the list elements from a child element defined within a parent element, if that is something useful that we'd like.

This can be used to validate if a child element is allowed to be nested within a parent element.