Setup binder as a Python package and publish pre-built wheels to PyPI

[haiiliin]

Description of the implementation

• The python package is placed in the python directory to avoid conflicts.
• The llvm-project branch v6.x is submoduled in python/llvm-project and is included in python/CMakeLists.txt with add_subdirectory.
• The source directory is symbolically linked to python/llvm-project/clang-tools-extra/binder in the llvm-project submodule.
• The version of the python package is automatically inferred from the git tag using setuptools-scm.
• scikit-build-core is used to build the Python package with cmake.
• cibuildwheel is used to build wheels on different platforms, and archs using GitHub actions, and they will be published to PyPI when a release is published.