feat: implement (basic) webidl & headers comments

[KhafraDev]

Fixes #933

WebIdl is pretty cool and it's used rather heavily in the fetch spec. Implementing a subset of it (that we need) will make everything much easier once it's finished. I can explain how it'll help if you want, just don't want to write out an essay if you don't care lol

I only did the Headers class as a PoC and in the process discovered a load of bugs and decided just to add the spec comments since I was already adding a ton to it and fixing stuff anyways 😄.

p.s.: it's very clear that HeadersList was not supposed to be Map-like structure, as each key can be duplicated and the actual appending doesn't occur until you use .get(...). Not a big deal, just something I noticed when working on it.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 97.22222% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.78%. Comparing base (8e9d902) to head (7bd99dc).
⚠️ Report is 1952 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/fetch/util.js	90.90%	1 Missing ⚠️
lib/fetch/webidl.js	94.11%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1495   +/-   ##
=======================================
  Coverage   94.78%   94.78%           
=======================================
  Files          49       50    +1     
  Lines        4293     4335   +42     
=======================================
+ Hits         4069     4109   +40     
- Misses        224      226    +2     

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[mcollina]

Could you add some tests for the new code?

[mcollina]

lgtm

[mcollina]

Could you add some tests for the new code?

[KhafraDev]

added tests

all new stuff should be tested with existing tests already (everything passed other than Symbol tests which shouldn't have worked), just added some tests for invalid/valid characters.

[mcollina]

lgtm