Add a basic sky-element custom element

sky/tests/modules/resources/multiple-scripts-child.sky

 <sky>
 <script>
 final String one = 'one';
 bool oneInit = false;
-void init() {
+void _init(_) {
     oneInit = true;
 }
 final conflict = 'one';
 </script>
 <script>
 final String two = 'two';
 bool twoInit = false;
-void init() {
+void _init(_) {
     twoInit = true;
 }
 final conflict = 'two';
 </script>
 </sky>