refactor: convert CLI module into package with modular structure

Convert vcs_versioning._cli from a single module into a well-organized package
with separate concerns for argument parsing, command execution, and resources.

Package Structure:
- vcs-versioning/src/vcs_versioning/_cli/__init__.py:
  - Main CLI entry point and command execution
  - Extracted _get_version_for_cli() helper function
  - Refactored print functions (print_json, print_plain, print_key_value)
  - Added PRINT_FUNCTIONS dispatch table
  - Updated to use CliNamespace for type safety
  - Simplified file finder import (no try/except)
  - Added assertion for archival_template (guaranteed by argparse)

- vcs-versioning/src/vcs_versioning/_cli/_args.py:
  - New module for CLI argument parsing
  - CliNamespace class: typed argparse.Namespace subclass
  - Type annotations for all CLI options
  - get_cli_parser() function moved from __init__.py

- vcs-versioning/src/vcs_versioning/_cli/git_archival_stable.txt:
  - Template for stable git archival file (no branch names)
  - Loaded as package resource

- vcs-versioning/src/vcs_versioning/_cli/git_archival_full.txt:
  - Template for full git archival file (with branch info)
  - Loaded as package resource

Key Changes:
- Argument parsing:
  - --stable/--full now store_const template filenames
  - parse_args() receives CliNamespace() instance for type safety

- Version handling:
  - Extracted into _get_version_for_cli() function
  - Returns str (not str | None) - simpler type
  - Exception handling (SystemExit) happens naturally

- Template handling:
  - Archival templates moved to resource files
  - Single loading point using importlib.resources.files()
  - Removed _get_stable_archival_content() helper
  - Removed _get_full_archival_content() helper

- Print functions:
  - Renamed _print_plain -> print_plain
  - Renamed _print_key_value -> print_key_value
  - Added print_json function
  - Dispatch via PRINT_FUNCTIONS dict
  - Accept MutableMapping instead of dict for flexibility

Benefits:
- Better code organization and separation of concerns
- Type safety with CliNamespace annotations
- Easier to test individual components
- Templates are maintainable resource files
- More data-driven design (template filenames, print dispatch)
- Prepares for future CLI feature additions
- No external dependencies (importlib.resources is Python 3.10+)

All 10 CLI tests pass.

Author: RonnyPfannschmidt