fix(fetch): missing body-mixin brand checks

[KhafraDev]

This adds brand checks to body mixin methods & properties. These are webidl checks done throughout the spec. This disallows the following:

// accessing these methods without a proper "this" value
Response.prototype.bodyUsed
Response.prototype.body
Request.prototype.bodyUsed
Request.prototype.body

Request.prototype.text.call({ some: 'this value' })
Response.prototype.text.call({ some: 'this value' })
// same for all other body mixin methods
// "this" must be a Request/Response object now.

These checks are already done everywhere else.

Refs: #1436 #1430

[codecov-commenter]

Codecov Report

Merging #1498 (89d97f4) into main (a1600f9) will decrease coverage by 0.09%.
The diff coverage is 97.95%.

@@            Coverage Diff             @@
##             main    #1498      +/-   ##
==========================================
- Coverage   94.74%   94.64%   -0.10%     
==========================================
  Files          49       49              
  Lines        4297     4312      +15     
==========================================
+ Hits         4071     4081      +10     
- Misses        226      231       +5     

Impacted Files	Coverage Δ
lib/fetch/body.js	98.51% <97.87%> (+0.18%)	⬆️
lib/fetch/request.js	85.89% <100.00%> (ø)
lib/fetch/response.js	93.33% <100.00%> (ø)
lib/client.js	97.63% <0.00%> (-0.66%)	⬇️

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[mcollina]

Could you clarify in the PR description what this does? It's a bit unclear:

[KhafraDev]

Done, sorry 😄

[mcollina]

lgtm