refactor(internal): cleanup `diff()`

[iuioiua]

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To improve maintainability. I wasn't able to make snake() top-level. Oh well. That can be done some other time.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.28571% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 91.47%. Comparing base (c53ca87) to head (b1773ef).

Files	Patch %	Lines
internal/diff.ts	94.28%	6 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4704      +/-   ##
==========================================
- Coverage   91.48%   91.47%   -0.01%     
==========================================
  Files         477      477              
  Lines       37273    37248      -25     
  Branches     5316     5311       -5     
==========================================
- Hits        34098    34072      -26     
- Misses       3119     3120       +1     
  Partials       56       56              

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