Add HTMLHyperlinkElementUtils

[chexxor]

The a and area elements implement the HTMLHyperlinkElementUtils interface [0]. How are mixin interfaces like this usually included in this repo? It's not a proper element, just an extracted interface, so we can't make a new type in DOM.HTML.Types and check the tag name like this:

-- We can't do this, because an `a` element returns tag name of "HTMLAnchorElement".
readHTMLHyperlinkElementUtils :: Foreign -> F HTMLHyperlinkElementUtils
readHTMLHyperlinkElementUtils = unsafeReadTagged "HTMLHyperlinkElementUtils

But we could do this:

htmlAnchorElementToHTMLHyperlinkElementUtils :: HTMLAnchorElement -> HTMLHyperlinkElementUtils
htmlAnchorElementToHTMLHyperlinkElementUtils = unsafeCoerce

and put that interface's members in a DOM.HTML.HTMLHyperlinkElementUtils module, like the href function. Should we just do that?

[0] https://html.spec.whatwg.org/multipage/links.html#htmlhyperlinkelementutils

[garyb]

Yep, unsafeCoerce is fine here - it's not an upcast, so it won't fail, and since it's an interface there's no corresponding class we can unsafeReadTagged for.