Add a “template mode”

[sellout]

I’ve done this manually in some cases, but what would be nice is to tell Project Manager that you want to effectively run it once, producing a set of files to commit without expecting Project Manager to be included at all.

project-manager switch --template

• this is different from project.commit-by-default = true, because “template mode” would ignore any settings on individual files
• this implies that no generated files would contain references to the Nix store

Stretch goals:

• generate files containing the flake outputs that Project Manager makes available, so they can still be used even if Project Manager isn’t there
• implement Restore non-flake support #16, since currently you would need Project Manager in your flake to generate the template, then remove it before committing
• possibly implied by the above, make it possible to reference a project configuration outside of the working tree, so that nothing needs to be deleted after generating the template