add bodymixin.textStream()

[KhafraDev]

refs: whatwg/fetch@30140d0

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.35%. Comparing base (cb105d7) to head (e471e9e).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5416      +/-   ##
==========================================
+ Coverage   93.26%   93.35%   +0.08%     
==========================================
  Files         110      110              
  Lines       36823    36928     +105     
==========================================
+ Hits        34344    34475     +131     
+ Misses       2479     2453      -26     

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[tsctx]

Could you add typing for this?