Support <optgroup> in <datalist>

[cawa-93]

This is fork from w3c/html#932

Original issue

<optgroup> is useful for displaying options to users, but browsers don't support it with <datalist>.

4.10.9 The optgroup element has only one context specified for <optgroup>

As a child of a select element.

This issue suggests adding another

As a child of a datalist element.

or in some other way specifying that browsers should support <optgroup> when displaying options on a <datalist> 🙏