Add numeric indexer to HTMLElementCollection

The change is adding ``` [index: number]: Element; ``` to HTMLCollection .

The definition of HTMLCollection has a string indexer, but we can not use it as it has a length property that is of type number; so we comment that out. 

there is no reason however to not include the numeric indexer.

the generation script moved it up, so this is why the change looks weird.
Mohamed explained it in the PR message, but I'm still a little confused, and I think it warrants a code comment.