Add support method for Function as Child Component Pattern

[subroh0508]

relates #130

Several functions have been added to support the Function as Child Component Pattern.

For example, by JSX

const ParentComponent = (props) => {
  return (
    <div>
      { props.parentText }
      { props.children("Child") }
    </div>
  )
}

const ChildComponent = (props) => {
  return (
    <div>
      { props.childText }
    </div>
  )
}

const App = () => (
  <ParentComponent parentText="Parent">
    { (text) => (<ChildComponent childText={ text }>) }
  </ParentComponent>
)

/* [Result]
 * <div>
 *   Parent
 *   <div>Child</div>
 *  </div>
 */

On this Pull Request, for example

external interface ParentProps : RProps {
    var parentText: String
}

external interface ChildProps : RProps {
    var childText: String
}

// Functional Component
val FunctionalParentComponent = functionalComponent<ParentProps> { props ->
    div {
        +props.parentText
        props.children("Child")
    }
}

val FunctionalChildComponent = functionalComponent<ChildProps> { props ->
    div {
        +props.childText
    }
}

fun RBuilder.app() = childFunction(FunctionalParentComponent, {
    attrs.parentText = "Parent"
}, { text: String ->
    child(FunctionalChildComponent) { attrs.childText = text }
})

// Class Component
class ParentComponent : RComponent<ParentProps, RState>() {
    override fun RBuilder.render() {
        div {
            +props.parentText
            props.children("Child")
        }
    }
}

class ChildComponent : RComponent<ChildProps, RState>() {
    override fun RBuilder.render() {
        div {
            +props.childText
        }
    }
}


fun RBuilder.app() = childFunction(ParentComponent::class, {
    attrs.parentText = "Parent"
}, { text: String ->
    child(ChildComponent::class) { attrs.childText = text }
})