feat(http): stabilize structured fields

[tomas-zijdemans]

Stabilizes the RFC 9651 Structured Field Values module (added in #6963).

• Renames unstable_structured_fields.ts → structured_fields.ts; export is now @std/http/structured-fields
• Renames the Parameters type to ParameterMap, as it shadowed TypeScript's built-in Parameters<T> utility type
• Removes @experimental tags and exports the module from mod.ts
• Updates unstable_message_signatures.ts imports accordingly

The module passes the official HTTPWG conformance suite and has had no behavioral changes since landing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.84%. Comparing base (0e5b1f2) to head (730a1f6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7207   +/-   ##
=======================================
  Coverage   94.83%   94.84%           
=======================================
  Files         617      617           
  Lines       51674    51674           
  Branches     9350     9350           
=======================================
+ Hits        49007    49008    +1     
  Misses       2121     2121           
+ Partials      546      545    -1     

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