refactor: add toolbox module

[mtth]

This will make it easier to extend (e.g. with diff support).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.50549% with 5 lines in your changes missing coverage. Please review.

Project coverage is 93.30%. Comparing base (becd123) to head (89c005a).

Files with missing lines	Patch %	Lines
src/git_draft/toolbox.py	93.05%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
- Coverage   93.36%   93.30%   -0.07%     
==========================================
  Files           8        9       +1     
  Lines         392      403      +11     
  Branches       32       32              
==========================================
+ Hits          366      376      +10     
- Misses         18       19       +1     
  Partials        8        8              

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