make formdata body immutable and encode it only once

[jimmywarting]

with this changes you only have to encode the formdata once.
it also creates an array of blob parts to make it immutable so that it can't be changed afterwards

This basically solves the following problem:

const fd = new FormData()
fd.set('foo', 'bar')
const res = new Response(fd)
fd.set('foo', 'foo')
// fd.set('foo', 'foooo') could also set the field to a other size making req content-length invalid.
const fd2 = await res.formData()
fd2.get('foo')

reason why i didn't just do action = new Blob(parts) was due to 3rd party files that can be backed up by fs

[codecov-commenter]

Codecov Report

Base: 87.27% // Head: 87.29% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (921e2aa) compared to base (860c261).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1814      +/-   ##
==========================================
+ Coverage   87.27%   87.29%   +0.01%     
==========================================
  Files          66       66              
  Lines        5792     5792              
==========================================
+ Hits         5055     5056       +1     
+ Misses        737      736       -1     

Impacted Files	Coverage Δ
lib/fetch/body.js	96.53% <100.00%> (ø)
lib/fetch/file.js	90.80% <0.00%> (+1.14%)	⬆️

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[KhafraDev]

Can you add a test for the immutable formdata?

[jimmywarting]

sure

[jimmywarting]

hmm, Wondering if i could add the test to WPT.

[KhafraDev]

It'd make more sense as a WPT, but I don't think there's a reason to hold up this PR waiting for the test to be accepted.