Add ESM support to WebC: swap to use import-module-string instead of node-retrieve-globals

[zachleat]

Fixes #226

• Breaking: Now requires an export default in both webc:type="render" and webc:type="js" nodes.
• Breaking: Drops support for CommonJS module.exports in webc:type="render"
• webc:type="render" and webc:type="js" now use the same code path behind the scenes and are interchangeable.
• More straightforward use of data: adds a default data argument to webc:type="js" and webc:type="render" default functions: export default function(data) {}
• Better ESM: adds support for both import and require to webc:type="render" and webc:type="js" nodes (previously only require was supported)

Examples of new export default requirements:

<script webc:type="js">
+export default function() {
  this.alwaysBlue(\`hello\`);
+}
</script>

<script webc:type="js">
+export default function(context) {
-  this.alwaysBlue(\`hello\`);
+  context.alwaysBlue(\`hello\`);
+}
</script>

<script webc:type="render">
-function() {
+export default function() {
  this.alwaysBlue(\`hello\`);
}
</script>