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.
I am not sure I understand the question.. the indexer is to allow collection[0] ==> Element, instead of an implicit any.
the change in order is about the script I use to generate the file, if I override any thing in the interface it process it firs.