Specs: make element registrations be per-module, define how they are

sky/examples/radio.sky

 SKY MODULE - radio button and radio button group
 <!-- accessibility handling not implemented yet, pending mojo service -->
-<import src="sky:core" as="sky"/>
+
+<script>
+ module.exports = {};
+</script>
 
 <!-- <radio> -->
 <template id="radio-shadow">
  <style>
   :host { width: 1em; height: 1em; border: solid; background: white; }
   :host[checked] { background: black; }
  </style>
 </template>
 <script>
- module.exports = {};
- module.exports.RadioElement = sky.registerElement({
+ module.exports.RadioElement = module.registerElement({
    tagName: 'radio',
    shadow: true,
    prototype: class extends Element {
      constructor () {
        super();
        this.addEventListener('click', (event) => this.checked = true);
        this.shadowRoot.appendChild(module.document.findId('radio-shadow').content.cloneNode(true));
      }
      get checked () {
        return this.hasAttribute('checked');
@@ skipping 19 matching lines @@
  });
 </script>
 
 <!-- <radiogroup> -->
 <template id="radiogroup-shadow">
  <style>
   :host { padding: 1em; border: thin solid; }
  </style>
 </template>
 <script>
- module.exports.RadioGroupElement = sky.registerElement({
+ module.exports.RadioGroupElement = module.registerElement({
    tagName: 'radiogroup',
    shadow: true,
    prototype: class extends Element {
      constructor () {
        super();
        this.shadowRoot.appendChild(module.document.findId('radiogroup-shadow').content.cloneNode(true));
      }
      get value () {
        let children = this.getChildNodes();
        for (let child of children)
@@ skipping 14 matching lines @@
          throw;
        let children = this.getChildNodes();
        for (let child of children)
          if (child instanceof module.exports.RadioElement)
            if (child != radio)
              child.checked = false;
      }
    },
  });
 </script>