feat(Headers): spec-compliant fill & constructor

[KhafraDev]

• Removes TODOs in the fill method. Weird behavior can be entirely explained with one word: "webidl".
• Fixes some invalid errors being thrown when HeadersInit is a function (in webidl, functions are considered Object types).
• Adds a webidl record<key, value> converter.
• Combined webidl.sequenceConverter and webidl.converters.sequence as the latter has no uses without the former.

p.s. once I make a pr for Response/Request idl, I am going to make a PR to improve the error messages.

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No major changes in this pr. All current tests pass (other than 2 which were invalid) and node-fetch tests pass.

[codecov-commenter]

Codecov Report

Merging #1502 (e0484a0) into main (8924332) will decrease coverage by 0.02%.
The diff coverage is 95.34%.

@@            Coverage Diff             @@
##             main    #1502      +/-   ##
==========================================
- Coverage   94.83%   94.80%   -0.03%     
==========================================
  Files          50       50              
  Lines        4512     4525      +13     
==========================================
+ Hits         4279     4290      +11     
- Misses        233      235       +2     

Impacted Files	Coverage Δ
lib/fetch/webidl.js	97.98% <92.85%> (-1.28%)	⬇️
lib/fetch/headers.js	94.96% <100.00%> (-0.04%)	⬇️

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