DataTransfer#types is incorrectly typed

[markschaake]

Noticed this as a runtime error. The offending line of code is here, which is:

def types: DOMStringList = js.native

I believe it should be instead:

def types: js.Array[DOMString] = js.native

See the w3c spec, and the MDN docs which both state the type is a string array.

I'll issue a PR shortly.