Add command to generate model

[hannobraun]

When someone wants to try Fornjot, there's not a great way to see something quickly, unless they're cloning the Git repository and use the example models in there. For that reason, and also to make it easier to get started in general, it would be great if the Fornjot app could generate an example model.

Eventually, this should be possible from within the app, but for now, a command-line argument would suffice. Maybe something like this:

fj-app model new <name>

This would generate a model in the <name>/ directory within the current one. The generated model should be something simple. cuboid or spacer could be good candidates. Maybe even star. It doesn't actually matter much. Once the infrastructure is in place, it should be easy to change which model is being generated.

Ideally, the generated model should be regularly tested, so it doesn't become stale. Maybe including one of the example models directly in the binary using something like include_dir would be an option. Those are part of the CI build, so they should always be in sync with the app.

Labeling https://github.com/hannobraun/Fornjot/labels/good%20first%20issue. This will require some figuring out and design decisions, but as far as Fornjot is concerned, it only requires surface knowledge. The place to add the command-line argument is args.rs in fj-app, and any of the code for generating the model can just go into a new module within fj-app.

[hannobraun]

I've just removed this feature again in #1373. See #1356 for context.

Reopening.

[hannobraun]

This issue is no longer applicable. The affected code has been removed. See A New Direction for context.