chore: Make AppInstance available as mixing

[codekeyz]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 37.50000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 71.52%. Comparing base (713f544) to head (277b325).

Files with missing lines	Patch %	Lines
packages/pharaoh/lib/src/_next/core.dart	50.00%	3 Missing ⚠️
packages/pharaoh/lib/src/core_impl.dart	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #134      +/-   ##
==========================================
- Coverage   71.56%   71.52%   -0.05%     
==========================================
  Files          45       45              
  Lines        1530     1531       +1     
==========================================
  Hits         1095     1095              
- Misses        435      436       +1     

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