Support React-like interface for functional components/elements

This allows JSX to render custom elements defined as `const
CustomElement = props = > <button
onclick={props.onclick}>{props.label}</button>`

Author: gnestor

packages/virtualdom/src/index.ts

@@ -649,6 +649,11 @@ type ElementSpecialAttrs = {
    * The inline style for the rendered DOM element.
    */
   readonly style?: ElementInlineStyle;
+  
+  /**
+   * The children attribute for functional custom elements.
+   */
+  children?: ReadonlyArray<VirtualNode>;
 };
 
 
@@ -1020,6 +1025,12 @@ namespace Private {
     if (node.type === 'text') {
       return document.createTextNode(node.content);
     }
+    
+    // If node is a function, call if with its attrs to return a VirtualNode.
+    if (typeof node.tag === 'function') {
+      node.attrs.children = node.children;
+      node = node.tag(node.attrs) as VirtualElement;
+    }
 
     // Create the HTML element with the specified tag.
     let element = document.createElement(node.tag);