Add PonyGE2 grammar parser

[alcides]

Summary

• Parse PonyGE2 BNF grammar text and files.
• Generate dynamic GeneticEngine nonterminal and production dataclasses.
• Return an extracted GeneticEngine Grammar and starting symbol.
• Support ordinary terminals through VarRange.
• Support GE_RANGE:n and named ranges via ge_constants.
• Add parser, error-handling, and tree-generation tests.

Closes #162

Validation

• Ruff passed.
• Mypy passed.
• Full test suite: 520 passed, 8 skipped.