refactor(internal): cleanup and restructure

[iuioiua]

This change simplifies the DiffType and moves functions out into their own respective files with the aim of improving maintainability, as internal/diff.ts is quite a busy file. Further improvements should come after this PR.

[codecov]

Codecov Report

Attention: Patch coverage is 93.56725% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 91.47%. Comparing base (883f129) to head (1d7bc24).
Report is 1 commits behind head on main.

Files	Patch %	Lines
internal/diff_str.ts	92.59%	8 Missing ⚠️
internal/build_message.ts	94.11%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4698      +/-   ##
==========================================
- Coverage   91.47%   91.47%   -0.01%     
==========================================
  Files         477      479       +2     
  Lines       37318    37315       -3     
  Branches     5325     5325              
==========================================
- Hits        34137    34134       -3     
  Misses       3125     3125              
  Partials       56       56              

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[kt3k]

LGTM