add import statements to @theory declaration

[kris-brown]

This allows us to explicitly declare that the generated methods in the associated theory module extend some other methods.

[olynch]

nice

[codecov]

Codecov Report

Patch coverage: 86.20% and project coverage change: -0.17% ⚠️

Comparison is base (15cec29) 96.84% compared to head (9117faf) 96.68%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #107      +/-   ##
==========================================
- Coverage   96.84%   96.68%   -0.17%     
==========================================
  Files          26       27       +1     
  Lines        1458     1477      +19     
==========================================
+ Hits         1412     1428      +16     
- Misses         46       49       +3     

Files Changed	Coverage Δ
src/stdlib/models/module.jl	100.00% <ø> (ø)
src/syntax/GATs.jl	95.61% <80.00%> (-0.49%)	⬇️
src/syntax/TheoryInterface.jl	96.42% <88.88%> (-1.45%)	⬇️
src/models/ModelInterface.jl	97.63% <100.00%> (ø)
src/stdlib/models/Arithmetic.jl	100.00% <100.00%> (ø)

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