DRAFT: rewrite c++ wrappers using nanobuild

This draft PR completely revamps the c++ library wrappers for antspy. It uses nanobuild which is a greatly improved version of pybind11.

The positive effects of this PR are the following:

all c++ code moves to its own src/ dir instead of ants/lib/ dir... having c++ code in the python directory is an anti-pattern and confused a lot of people
all lib code hidden from users.. almost no chance they accidentally import it
wheel size greatly decreased. i estimate it will be between 50 - 100 mb instead of 250+ mb now.
much faster to import antspy in the console. with the existing pybind11 wrappers it takes 30+ seconds the very first time antspy is installed and maybe 5 seconds each subsequent time. this will drop that to an estimated <15 seconds and <2 seconds to import. it's very quick.
all pixeltypes and dimensions for itk will be supported. previously we had to compromise because more pixeltypes meant much larger build sizes. Now that this moves away from the templating paradigm, we can support everything
on that note... greatly simplified library interface. previously there were a million different library calls for the same function (e.g., lib.ThresholdImageF2(img), lib.ThresholdImageF3(img), etc... now it will be one function, e.g. lib.ThresholdImage(img, 'F2')
uses modern built system (pyproject.toml) instead of relying solely on setup.py

Potential negative effects:

unsure how this will affect windows builds or if it will make it harder for people to install antspy... however, we are not perfect at that anyways right now
CI / GHA workflows may need to be reworked a bit

Current difficulties:

Because this uses a very modern build system, I have NO idea how to run prebuild bash scripts to configure ITK and ANTs. I have added those bash calls into the CMakeLists.txt file for now but I'd like a better solution. The current method we use of calling two bash scripts in setup.py seems like a hacky anti-pattern though, so ideally ITK would be built together with this project (like a superbuild?). I have tried a ton of things with no success.

Testing this:

pip install .

ANTsX / ANTsPy