install-path XDG

There should be a way to tell install-path to try to use XDG dirs (quick skimming suggests that just means ` $HOME/.local/bin`?). What exactly that means on windows is contentious, but if we have #286 we can just say "this doesn't make sense on windows" and ignore it in the cascade.

Start at the install-path entry point and read issue #286 for the proposed Windows handling. Trace the existing cascade, determine how XDG directories should be attempted on supported platforms, and confirm the behavior is explicitly ignored on Windows; done means install-path can select the XDG location without changing the Windows cascade.

Written by the indexing model from the issue text.