Adds `basename` and `dirname` interpolation. I want to add a `stack` tag to our infrastructure, the value of which is set to `${basename(path.cwd)}`. We currently use `${replace(path.cwd, "/^.+\\//", "")}` instead, but this is extremeley unreadable. The existance of a `basename` function would be very useful for this use case.
I don't have an immediate use case for a `dirname` function, but it seemed reasonable to add it as well.