feat: Add comprehensive codecs module with extension-based codec registries

Implements Pattern 1 (extension-based codec registries) for flexible
configuration file parsing with support for bytes <-> JSON-friendly Python types.

New features:
- Core registry infrastructure with EXTENSION_TO_DECODER/ENCODER mappings
- Registration API: register_codec(), register_decoder(), register_encoder()
- Introspection API: list_registered_extensions(), is_extension_registered(), get_codec_info()
- encode_by_extension() and decode_by_extension() for automatic codec routing

Standard library codecs (always available):
- JSON (.json) - with proper UTF-8 and indentation
- Pickle (.pkl, .pickle) - Python object serialization
- Plain text (.txt)
- Line-separated values (.lsv, .list, .lines)
- CSV (.csv) - with dict support
- TSV (.tsv, .tab) - tab-separated values
- INI/CFG (.ini, .cfg, .conf) - ConfigParser format
- XML (.xml) - basic ElementTree support

Conditional third-party codecs:
- TOML (.toml) - tomllib (3.11+) / tomli + tomli_w
- YAML (.yaml, .yml) - PyYAML with safe_load
- ENV (.env) - python-dotenv with simple fallback
- JSON5 (.json5) - lenient JSON parsing
- Properties (.properties) - Java-style with jproperties fallback

Extras added to pyproject.toml:
- Individual extras: yaml, toml, env, json5, properties
- Convenience groups: all-codecs, all

Key implementation details:
- Graceful fallback for missing third-party dependencies
- Dotfile support (e.g., .env, .gitignore)
- Comprehensive test suite with 100+ test cases
- Full type hints and docstrings

Author: claude

config2py/__init__.py

@@ -24,3 +24,4 @@
     process_path,
 )
 from config2py.sync_store import SyncStore, FileStore, JsonStore, register_extension
+from config2py import codecs  # noqa - Make codecs module available