Create monorepo

[gdalle]

Put DifferentiationInterface and DifferentiationInterfaceTest side by side, following the discussion on Discourse (https://discourse.julialang.org/t/anything-to-be-wary-of-when-registering-a-subpackage/112459)

Doc previews:

• DifferentiationInterface: https://gdalle.github.io/DifferentiationInterface.jl/DifferentiationInterface/previews/PR136/
• DifferentiationInterfaceTest: https://gdalle.github.io/DifferentiationInterface.jl/DifferentiationInterfaceTest/previews/PR136/

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.35%. Comparing base (2ee1bf1) to head (e89e664).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #136      +/-   ##
==========================================
- Coverage   95.96%   89.35%   -6.62%     
==========================================
  Files          59       16      -43     
  Lines        1860      808    -1052     
==========================================
- Hits         1785      722    -1063     
- Misses         75       86      +11     

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