Support optional installation of pyobjc packages by dependency

`pip` supports dependencies necessary on specific operating systems, [written like this](https://stackoverflow.com/a/75711133). I thought conda does as well, however at the very least we could install libraries optionally via pip within the conda environment file.

We should use these platform-specific dependencies to install pyobjc on mac, so that you can use gui mode right away.

Start by locating the conda environment file and reviewing how its dependencies are declared. Add the platform-specific installation for pyobjc on macOS, then verify that a macOS conda environment installs it and supports GUI mode.

Written by the indexing model from the issue text.