Add Option to mypyc to only Generate and Output C Code

[srilman]

I'm using scikit-build-core to build a fairly large Python project with multiple C++ files. There are some Python files that I want to optimize, so I'm using Cython. scikit-build-core doesn't have a special plugin for Cython like setuptools, so I'm just using the CLI. Something like:

cython -3 --output-file ${CMAKE_CURRENT_BINARY_DIR}/pkg pkg/module.py

This generates a .c & .h file that CMake can keep track of. I wanted to migrate some of these modules to MyPyC since I'm seeing better performance, but there isn't a CLI option to do something similar. I was able to hack something together like:

python -c "from mypyc.build import mypycify; mypycify(['pkg/module.py'])"

But it would be helpful if I could do the same from the mypyc command. Maybe adding a subcommand or different option to specify that we only need the C file, and where to put it? What do y'all think?