MAINT: Add functions to parse shell-strings in the platform-native way

There are places in distutils where we accept a single string from the user, and interpret it as a set of command line arguments.
Previously, these were passed on as a string unmodified to exec_command, and interpreted by subprocess in a platform-specific way.

Recent changes to distutils now pass a list of arguments to subprocess, meaning we have to split the strings ourselves.
While `shlex.split` is perfect on posix systems, it is not a good approximation of either the old or the expected behavior on windows.

Provides the building blocks needed to fix gh-12979

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