feat: implement FormData Iterator

[KhafraDev]

Continuation of #1431 and #1423

Also adds in FormData.forEach which was added in the types but for some reason wasn't implemented.

[codecov-commenter]

Codecov Report

Merging #1473 (b74198f) into main (4711982) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1473   +/-   ##
=======================================
  Coverage   94.70%   94.70%           
=======================================
  Files          49       49           
  Lines        4246     4249    +3     
=======================================
+ Hits         4021     4024    +3     
  Misses        225      225           

Impacted Files	Coverage Δ
lib/fetch/formdata.js	96.77% <100.00%> (+0.07%)	⬆️
lib/fetch/headers.js	94.44% <100.00%> (-0.26%)	⬇️
lib/fetch/util.js	81.11% <100.00%> (+1.59%)	⬆️

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[mcollina]

lgtm

[kibertoad]

What's missing before this can be merged?

[KhafraDev]

A review from ronag I presume, who is currently on vacation 😄

[mcollina]

A review from ronag I presume, who is currently on vacation 😄

pretty much

[jimmywarting]

Hmm, tried doing export const FormData = globalThis.FormData || polyfill but notices that hour tests failed failed cuz this forEach was missing...

was using NodeJS 18.3.0

have a resent nodejs release been released with this fix?

[KhafraDev]

@jimmywarting v18.5.0 has FormData.prototype.forEach.