proposal(http/unstable): support RFC 9530 Want-Content-Digest / Want-Repr-Digest fields

#7238 adds an unstable `@std/http/unstable-digest-fields` module for creating and verifying RFC 9530 `Content-Digest` and `Repr-Digest` field values.

RFC 9530 §4 also defines the `Want-Content-Digest` and `Want-Repr-Digest` fields, which let a sender express a preference for which digest algorithms the peer should use (as an Integer Preference dictionary, e.g. `Want-Repr-Digest: sha-512=3, sha-256=10`). These were deliberately scoped out of #7238 to keep the initial module small.

This issue tracks adding helpers for creating and parsing the two `Want-*` fields, e.g.:

- serializing an algorithm-preference map to a field value
- parsing a received field value into an ordered preference list, ignoring unsupported algorithms per the RFC

Raised in review: https://github.com/denoland/std/pull/7238#discussion_r3681734720

Start by reading the existing @std/http/unstable-digest-fields module from #7238 and RFC 9530 §4. Trace how the current helpers create and verify digest fields, then define the corresponding Want-* serialization and parsing behavior, including ordered preferences and unsupported algorithms. Done means both fields are supported with behavior matching the RFC.

Written by the indexing model from the issue text.