Add a DiffReader for diffing two io.Readers

[FollowTheProcess]

Summary

[codecov]

Codecov Report

❌ Patch coverage is 63.63636% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.73%. Comparing base (36e6783) to head (07c7ae6).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
test.go	63.63%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
- Coverage   87.37%   86.73%   -0.65%     
==========================================
  Files           3        3              
  Lines         396      407      +11     
==========================================
+ Hits          346      353       +7     
- Misses         31       33       +2     
- Partials       19       21       +2     

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