Set the CWD of the command

I suppose I could use the technique of running `cd DIR` and hiding that cmd from the output, but thought it's be nice to have a higher order `Set` for the CWD as well?

No file, test, or entry point is named in the issue. Start by locating VHS command parsing and existing higher-order commands, then determine how a CWD-setting command should be represented without printing a cd command; done means a command can set the working directory invisibly.

Written by the indexing model from the issue text.