feat(Headers): implement Headers Iterator

[KhafraDev]

See:

• The new Headers implementation should be reverted node-fetch/node-fetch#1119 (comment)
• https://bugs.chromium.org/p/chromium/issues/detail?id=793204

This implements Headers Iterator which is a generator-like... thing... that only has a .next() method. Chrome doesn't implement this entirely correct so I recommend using Firefox to see the correct behavior.

Notes:

• Typescript's Header types are incorrect! You can't use .return() or .throw() on a Headers iterator (.keys(), .entries(), .values(), Symbol.iterator()).
• Deno does implement this correctly.
• The Headers standard doesn't mention Headers Iterator at all so I made this implementation through ✨ trial and error ✨.

[codecov-commenter]

Codecov Report

Merging #1423 (3f85887) into main (740053f) will increase coverage by 0.00%.
The diff coverage is 87.50%.

@@           Coverage Diff           @@
##             main    #1423   +/-   ##
=======================================
  Coverage   94.27%   94.27%           
=======================================
  Files          49       49           
  Lines        4244     4248    +4     
=======================================
+ Hits         4001     4005    +4     
  Misses        243      243           

Impacted Files	Coverage Δ
lib/fetch/headers.js	93.42% <87.50%> (+0.17%)	⬆️

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[mcollina]

lgtm