[whatwg/url] Define domain label (#435)

As identified in #434 we need to talk about domain labels from first principles somehow. There's various RFCs:

* https://tools.ietf.org/html/rfc1034#section-3.1 and https://tools.ietf.org/html/rfc8499, but these are problematic as they are byte-based.
* https://tools.ietf.org/html/rfc5890#section-2.3.2.1 is probably closest, but has some talk about validity that I'm not sure apply given we use different validation rules from IDNA2008...

Perhaps split domain on "." is best, since any relevant normalization will have already happened at that point (where we have determined it's a domain). And then reference some RFCs non-normatively.

See also #396.

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