Expand peptide modifications and add custom registries

[PerinMu]

Summary

• expand built-in residue, terminal, and backbone modification representations with site-aware Unimod aliases
• add layerable JSON modification registries, Python registration APIs, schema validation, override protection, and examples
• make the GitHub homepage fully English and link a complete Simplified Chinese README
• document attachment semantics, scientific conventions, provenance, and contribution requirements
• bump the package metadata to 0.2.0

Validation

• ruff check .
• pytest: 52 passed
• built moltranscode-0.2.0-py3-none-any.whl
• exercised formats and custom L/D peptide conversion through the CLI