feat(http/unstable): implement RFC 9651 Structured Field Values

[tomas-zijdemans]

Implements RFC-9651 parsing and serialization. RFC 9651 obsoletes RFC 8941 and is fully backwards compatible, adding support for Date and Display String types.

• Single file implementation (like http/user_agent.ts)
• Browser compatible
• Matching RFC 9651 terminology as much as possible
• Has all RFC 9651 types, including dates and display strings that extends RFC 8941
• Official HTTPWG conformance test suite included in testdata/structured_fields/ (JSON files from https://github.com/httpwg/structured-field-tests)

[codecov]

Codecov Report

❌ Patch coverage is 94.70672% with 37 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.09%. Comparing base (007896d) to head (1fb31c7).

Files with missing lines	Patch %	Lines
http/unstable_structured_fields.ts	94.70%	36 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #6963    +/-   ##
========================================
  Coverage   94.09%   94.09%            
========================================
  Files         600      601     +1     
  Lines       43565    44264   +699     
  Branches     6999     7202   +203     
========================================
+ Hits        40991    41652   +661     
- Misses       2519     2556    +37     
- Partials       55       56     +1     

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[tomas-zijdemans]

Test coverage is not 100%, this is due to defensive programming. In practise I would argue it's 100%.

Please advise if I need to address the patch test coverage.