Package jwt_tool as an installable CLI (pyproject + entrypoint)

[Trigus42]

Summary
Modernizes distribution so jwt_tool can be installed/run as a console script, updates install docs accordingly, and makes Windows colour support optional.

Changes

• Adds Python packaging via pyproject.toml with console script entrypoint (jwt-tool) and optional dependency group for Windows colours.
• Refactors jwt_tool.py to expose a main() function and keep __main__ behavior.
• Auto-enables colorama when available (no hard dependency).
• Updates installation/usage guidance in README.md and setup.txt for uv/pip.
• Adds lockfile and housekeeping: uv.lock, .gitignore, .python-version.

Notes

• requirements.txt is removed in favor of pyproject.toml.
• Windows colour support is now opt-in via .[win-colour].