feat(mixins): add mixins

[mnboos]

This is the PR with the mixin-functionality as shown in #278

[eadwinCode]

@mnboos This is amazing, well done. But is there a written test for this feature?

[mnboos]

Thank you!

No,.not yet. I thought I will do this, if you basically liked it. Which seems to be the case. So, shall I write tests and docs for it? I guess I shall. 😁

[mnboos]

I added tests and docs.

Coverage:

___________________________________ coverage: platform linux, python 3.12.8-final-0 ____________________________________

Name                                               Stmts   Miss  Cover   Missing
--------------------------------------------------------------------------------
...
ninja_extra/mixins/__init__.py                       174      0   100%
...
--------------------------------------------------------------------------------
TOTAL                                               2990     56    98%
========================================== 454 passed, 87 warnings in 12.26s ===========================================

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.15%. Comparing base (6a592c1) to head (64cd298).
Report is 9 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #279      +/-   ##
==========================================
+ Coverage   98.01%   98.15%   +0.14%     
==========================================
  Files          68       69       +1     
  Lines        2770     2987     +217     
==========================================
+ Hits         2715     2932     +217     
  Misses         55       55              

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