HTMLSelectElement doesn't support `selectedOptions` property

[AndrewKushnir]

The selectedOptions property is not supported on the select elements in Domino based on the definition here: https://github.com/fgnass/domino/blob/12a5f67136a0ac10e3fa1649b8787ba3b309e9a7/lib/htmlelts.js#L959-L979

The selectedOptions property is generally supported in all modern browsers and it'd be great to have a support in Domino as well. An absence of this support requires including extra code (like this one) into app bundles, thus increasing it unnecessarily.