[w3c/webcomponents] Allow custom `input` element content (#659)

Input has an agent shadow where can't access by javascript, But there anyway to access it or apply custom template for it, can make it works with custom content?
As there we can use file api to access file, a lot way to create custom input, why do not open the api to make easy to custom the build in element?
```javascript
this.shadowRoot.querySelector('#calendar');
```
or
```html
<input type="datetime">
    <template slot="calendar"></template>
</input>
```

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