[webcomponents] [Custom Elements] Import module (#329)

It will be very useful to describe the import attribute in custom element.

Example: index.html
```HTML
<custom-button import="MyClassButton from '/url/buttons.html'"></custom-button>
```

Example: /url/buttons.html'
```HTML
<style>
...
</style>
<script>
export class MyClassButton extends HTMLButtonElement
{
... Synchronous constructor and other API
}
</script>
```

User agent, automatically loading file and register element.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/329