Allow for both a handler and nested interactions on a single interaction node

[jbpros]

Consider the following:

interactions({
  ProjectInfoAboutProject: { named: (projectName) => { /* handler stuff */ } }
}) 

Say we also want a sub-interaction like this:

ProjectInfoAboutProject.named(projectName).hasCollaboratorNamed('Joe')

Is not feasible at the moment. It'd be good to have a similar API to tasks:

interactions({
  ProjectInfoAboutProject: { 
    named: interaction('projectName', ({ projectName }) => /* handler stuff */, { 
      hasCollaboratorNamed: interaction(
        'collaboratorName', 
        ({ projectName, collaboratorName }) => /* sub-handler */
      )
    })  
  }
})