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.
:) yes..

> cause we can not enable it. yet the definition says it has a string indexer. 
> so it is more of a guide to know what to expect if you index with a string and get any.

so it is included intentionally, as it reflects the spec, yet it is not a valid TypeScript definition.