fix(body mixin): only allow Uint8Array chunks

[KhafraDev]

I was writing a regression test for nodejs/node#43838 and discovered that the body mixin methods were still not spec-compliant. According to the stream standards, every chunk must be a Uint8Array when reading a body.

1. body-mixin .text() (for example), calls consume body
2. consume body calls fully reading body as promise
3. fully reading body as promise calls ReadableStreamDefaultReader read all bytes
4. and then read loop is called
5. read loop chunk steps says "If chunk is not a Uint8Array object, reject promise with a TypeError and abort these steps."

Writing this out because it's very convoluted any might help someone understand this in the future.

p.s.: .formData() likely isn't correct still, but I left it untouched.

[codecov-commenter]

Codecov Report

Merging #1550 (dd80fec) into main (6c9e634) will decrease coverage by 0.01%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##             main    #1550      +/-   ##
==========================================
- Coverage   94.91%   94.89%   -0.02%     
==========================================
  Files          50       50              
  Lines        4678     4686       +8     
==========================================
+ Hits         4440     4447       +7     
- Misses        238      239       +1     

Impacted Files	Coverage Δ
lib/fetch/body.js	97.59% <75.00%> (-1.15%)	⬇️
lib/fetch/file.js	92.85% <0.00%> (+1.19%)	⬆️

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[mcollina]

lgtm