Add backend functionality to the core

[bvdmitri]

This PR introduces "backends" to the GraphPPL framework, allowing external packages to connect with it and tweak specific node behaviors. Essentially, it lets us define aliases for nodes outside of GraphPPL, and also gives us the flexibility to adjust model creation based on certain flags in a backend.

Included in this PR tests are showcasing how this functionality works, like setting up aliases for custom "distributions".
The backend is also allowed to add/remove custom syntax transformation pipelines to the model parsing.

For user interaction, I've added DefaultBackend, which smoothly handles objects from Distributions.jl using the existing extension setup. New backends can build off this functionality, as demonstrated by the backend implemented in testutils.jl.

One thing to note is that the @model macro isn't exported anymore. Packages relying on it will need to import it explicitly or define their own with a different backend.

Additionally, this PR refines many old tests by incorporating the latest changes, such as using ModelGenerator consistently.

[bvdmitri]

Probably noise again, but better to double check

[codecov]

Codecov Report

Attention: Patch coverage is 90.78947% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 91.48%. Comparing base (4bc587a) to head (248e24d).
Report is 1 commits behind head on dev-4.0.0.

Files	Patch %	Lines
src/graph_engine.jl	83.72%	7 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           dev-4.0.0     #174      +/-   ##
=============================================
- Coverage      91.74%   91.48%   -0.26%     
=============================================
  Files             14       15       +1     
  Lines           1781     1774       -7     
=============================================
- Hits            1634     1623      -11     
- Misses           147      151       +4     

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