This removes some ambiguity about what the return value should be if
the index is out of range.
Previously, we would sometimes return a JS null, and other times a JS
undefined.
It will also let us fold together the checks for whether an index is a
supported property index, followed by getting the value just afterwards.
This was an unfortunate artifact from development. I originally added
this class in the DOM namespace alongside HTMLCollection until I
realized it was defined by the HTML spec instead. To remind myself to
move it over to the HTML namespace, I left this comment. It was moved
to the correct namespace before upstreaming to the main repo, but I
forgot to remove this comment!
This collection has some pretty strange behaviour, particularly with the
IsHTMLDDA slot which is defined in the javascript spec specifically for
this object.
This commit implements pretty much all of this interface, besides from
the custom [[Call]].
There is also no caching over this collection. Since it is a live
collection over the entire document, the performance is never going to
be great, and I am not convinced any speedup for this legacy interface
is worth a massive cache.
