Swap-env

- Upgrade dependencies
- Migrate from poetry to uv

Python requirement downgraded from 3.10 to 3.9.

**Full Changelog**: https://github.com/benberryallwood/swap-env/compare/v1.0.0...v1.0.1

swap-env` now searches upwards from the current directory for the first `.swap-env/` directory it finds, defaulting to `~/.swap-env/`, so users can store different `.env` files for different projects and use the same names for them.

Add prompt to save existing .env before overwriting (unless it's a symlink)