generate an initial pixi.toml from detected dependencies

[betolink]

Scenario: we have a python script or a Jupyter notebook, they import a series of dependencies from either conda or pip. It would be nice if Pixi could use something like https://github.com/bndr/pipreqs to scan a script or location and initialize our pixi.toml with these dependencies, maybe unbounded since we don't know the exact versions?

pixi scan script.py --init 
pixi scan ./ --init