Add pixi.toml and pixi.lock.

[oursland]

Adds the pixi environment files.

pixi is similar to npm for nodejs in that it combines Python package dependencies, package lock files, and tasks. The packages are those hosted on conda-forge, and the lock file describes the versions and dependencies for each of the platforms (linux-64, linux-aarch64, osx-64, osx-arm64).

To use:

1. Install pixi as per the installation instructions.
2. Install the environment with pixi install
3. Run the application pixi run app

Adding new dependencies is easy with pixi add [dependency]

Updating the versions of dependencies in the lock file is performed with pixi update

Adding a new task is performed with pixi task add [name] [command]