feat(http/unstable): add parseRetryAfter

[tomas-zijdemans]

Adds parseRetryAfter to @std/http as an unstable API. It turns a Retry-After header value into milliseconds to wait, so a retry loop after a 429 or 503 can honor the server instead of guessing.

Implements the parser half of #7296. The predicate half is #7298.

What it does

• Accepts delta-seconds and all three RFC 9110 HTTP-date forms: IMF-fixdate, RFC 850, and asctime.
• Returns milliseconds from now, clamped to 0 for past dates. Returns null for anything invalid. Never throws.
• Skips Date.parse on purpose: it accepts junk like "-1" and " 120" and silently normalizes invalid calendar dates. Each grammar is matched exactly, with calendar and weekday validation.

Worth a close look

• The RFC 850 two-digit-year rule: a timestamp more than 50 years in the future resolves to the most recent past year with the same final two digits (RFC 9110 5.6.7).
• Leap seconds: second 60 normalizes to the following instant, since JS Date cannot represent it.
• Overflow: delta-seconds convert only when the millisecond result is a safe integer.

Scope

This PR only parses the field. Emitting Retry-After belongs to the server-side rate-limiting work in #7237. Honoring it inside @std/async retry() is a separate upcoming proposal. Nothing here touches @std/async.

Testing

23 tests with fixed now values and GMT dates: all three date forms, both sides of the 50-year boundary, a leap second, overflow, and a pile of near-miss syntax. deno task ok and the Node cross-runtime runner pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.05%. Comparing base (ca58f94) to head (3ab4731).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7299      +/-   ##
==========================================
+ Coverage   95.03%   95.05%   +0.01%     
==========================================
  Files         617      619       +2     
  Lines       51637    52030     +393     
  Branches     9359     9431      +72     
==========================================
+ Hits        49075    49455     +380     
- Misses       2021     2030       +9     
- Partials      541      545       +4     

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