Add a justfile to perform project install, test, clean, format tasks

[paulcadman]

This PR adds a https://github.com/casey/just

$ just
Available recipes:
    build *opts  # Build the project. `build runtime` builds only the runtime.
    clean *opts  # Clean the project. `clean runtime` cleans only the runtime. `clean juvix-build` cleans only juvix-build dirs.
    format *opts # Formats all Haskell files in the project. `format changed` formats only changed files. `format FILES` formats individual files.
    install      # Install juvix
    test *filter # Run the tests in the project. Use the filter arg to set a Tasty pattern.

[janmasrovira]

🎉