Type for `DataTransferItem.kind` is broader than implied by spec

[giraugh]

The type for the kind field on DataTransferItem is currently string however the spec suggests the more narrow literal type of "string" | "file". Possible also including the empty string.

DataTransferItem.kind (HTML Drag and Drop) Returns the drag data item kind, one of: "string", "file".

Happy to open a PR if this is a desired change :thumbsup: Cheers!

[andogq]

I've also encountered this discrepancy. Would be eager to see some progress on this!