NamedNodeMap is indexable by string

[evmar]

According to the spec (https://dom.spec.whatwg.org/#interface-namednodemap) and experimentally in Chrome, you can index a NamedNodeMap with a string and get back an Attr.

The TS definition only has:

interface NamedNodeMap {
    // [...]
    [index: number]: Attr;
}

[Bashamega]

This should be closed @saschanaz