Add forwarding behavior functions for accessibleName and helpText

[jessegreenberg]

A common use case for accessibleNameBehavior is to forward the accessibleName to child Node. The pattern for doing that is a bit verbose. It would be nice to factor some of this out:

    node.accessibleNameBehavior = ( node: Node, options: ParallelDOMOptions, accessibleName: PDOMValueType, callbacksForOtherNodes: ( () => void )[] ) => {
      callbacksForOtherNodes.push( () => {
        otherNode.accessibleName = accessibleName;
      } );
      return options;
    }

[jessegreenberg]

I started using these in a few components, I think they will be helpful. Closing.