feat: Introduce pseudoHeaders

[tsctx]

This relates to...

Fixs #2415

Rationale

Changes

See #2415

Features

Bug Fixes

Breaking Changes and Deprecations

Status

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• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
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[codecov-commenter]

Codecov Report

Attention: 26 lines in your changes are missing coverage. Please review.

Comparison is base (e39a632) 85.54% compared to head (8bd9655) 85.81%.
Report is 90 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2428      +/-   ##
==========================================
+ Coverage   85.54%   85.81%   +0.26%     
==========================================
  Files          76       76              
  Lines        6858     6921      +63     
==========================================
+ Hits         5867     5939      +72     
+ Misses        991      982       -9     

Files	Coverage Δ
index-fetch.js	100.00% <100.00%> (ø)
lib/api/api-stream.js	100.00% <100.00%> (ø)
lib/core/connect.js	81.25% <100.00%> (ø)
lib/fetch/body.js	97.68% <100.00%> (ø)
lib/fetch/constants.js	97.22% <100.00%> (+0.67%)	⬆️
lib/fetch/file.js	95.45% <100.00%> (+0.05%)	⬆️
lib/fetch/global.js	100.00% <ø> (ø)
lib/fetch/response.js	97.04% <100.00%> (+1.20%)	⬆️
lib/fetch/util.js	86.53% <100.00%> (+0.71%)	⬆️
lib/fetch/webidl.js	94.50% <100.00%> (-0.03%)	⬇️
... and 10 more

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[tsctx]

This seems a little too early.