AI-GCS: Design and implement "Code Stemming", e.g., token replacement and abstraction

[pombredanne]

This is something done here for instance https://dickgrune.com/Programs/similarity_tester/
See:

• https://github.com/pombredanne/sim/blob/master/project/javalang.l

Each code token is assigned a normalized replacement (here a single letter) and this is what is used afterwards.

Eventually this is to implement the "code stemming" as documented in https://ai-gen-code-search.readthedocs.io/en/latest/approximate-matching-design-3.html and https://github.com/aboutcode-org/ai-gen-code-search/blob/main/docs/source/approximate-matching-design-3.rst