[whatwg/fetch] Use structured fields for CORS headers (#1216)

Private Network Access is considering using structured fields for the new `Access-Control-Allow-Private-Network` header in https://github.com/WICG/private-network-access/issues/45. This header should be kept consistent with the existing `Access-Control-Allow-Credentials` header defined by the CORS protocol, since they both accept a single value: `"true"`.

It would be nice to modernize the existing ABNF-defined CORS header syntax to use structured fields instead.

To avoid backwards-incompatibility, the `Allow-Credentials` header in particular should probably be defined as a token instead of a boolean, which is unfortunate but surmountable.

It is less clear what to do with the `Access-Control-{Request,Allow}-{Method,Headers}` headers. Their syntax might be subtly different from that expected by structured fields' "list of tokens" type?

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