[directory-upload] This should be part of whatwg/html (#15)

Defining this standalone seems to miss all kinds of subtleties that would be more self-evident if this were a PR to whatwg/html. In particular the interaction with all the other input states is rather hand wavy, but there's also cause for concern with future updates to `<input type=file>.files`.

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Reply to this email directly or view it on GitHub:
https://github.com/WICG/directory-upload/issues/15