Implement the `use_python_app_entrypoint`

[wolfv]

Was osx_is_app in conda-build

This is a seldomly used option that changes the shebang to python.app instead of python on macOS .

I am not sure what it does exactly, but conda-forge uses it in a few places: https://github.com/search?q=org%3Aconda-forge+osx_is_app+language%3AYAML&type=code

build:
  python:
    # Specifies if python.app should be used as the entrypoint on macOS
    # was `osx_is_app`
    use_python_app_entrypoint: bool (defaults to false)  # macOS only!

[wolfv]

This also adds a requirement into the run dependencies (python.app) which can be found here;

https://github.com/conda-forge/python.app-feedstock/blob/main/recipe/meta.yaml

From conda-build:

    def append_requirements(self):
        """For dynamic determination of build or run reqs, based on configuration"""
        run_reqs = self.meta.setdefault("requirements", {}).setdefault("run", [])
        if (
            self.get_value("build/osx_is_app", False)
            and self.config.platform == "osx"
            and "python.app" not in run_reqs
        ):
            run_reqs.append("python.app")