fix: resolve Python 3.14 SegFault and modernize PyO3 implementation

[ElissonRodrigues]

Summary

This PR resolves the Segmentation Fault occurring on Python 3.14.3 during stemmer initialization and modernizes the overall Rust-Python interface. It also introduces type stubs for better developer experience.

Context

This PR addresses the root cause of the Segmentation Fault reported in qdrant/fastembed#618 when running on Python 3.14. While the issue was observed in fastembed, the underlying crash occurs within the py_rust_stemmers extension during initialization.

Key Changes

• Python 3.14 Compatibility: Upgraded to PyO3 0.28.2 and implemented the Python Stable ABI (abi3-py38). This ensures long-term stability across Python versions without requiring recompilation for each release.
• Robust Error Handling: Migrated from panic! into PyResult<Self> in the SnowballStemmer constructor. Invalid language inputs now raise a standard ValueError in Python instead of crashing the process.
• Modern Bound API: Refactored the implementation to use the modern PyO3 Bound API, satisfying stricter trait requirements and improving memory safety.
• IDE Support: Added Type Stubs (.pyi) and the py.typed marker (PEP 561) to enable autocompletion and type checking in IDEs and linters.
• Test Suite Restoration: Fixed the unit test suite and resolved a TypeError in speedtest.py related to Python 3 string/bytes handling in the comparison baseline.

Verification

• Manually verified on Python 3.14.3 (Linux/WSL).
• All unit tests pass (tests/test_py_rust_stemmers.py).
• Benchmarks (speedtest.py and benchmark_for_quantile.py) verified for performance parity.

Performance

The Rust implementation remains ~35x-40x faster than the pure Python snowballstemmer baseline on modern large-scale benchmarks.