instead of being os-agnostic, interpret the os-native expansion style,
so it is consistent with proper (shell) command lines.

don't interpret quotes, as this function is meant for expanding isolated
filepaths, where nobody would expect quoting. instead, use the windows
style of simply not doing an expansion if a referenced variable is not
found, which should be good enough - it's rather unlikely that something
which happens to be an expansion of an existing variable is actually not
meant to be one.