Add a `scripts` option that places scripts in `$XDG_DATA_HOME/darkman/`,
darkman's unified script interface. The legacy `darkModeScripts` and
`lightModeScripts` options remain supported.
Closes#8940
Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
