DSC: dsc_script resource

@btm, @jdmundrawala, here's a version of the DSC stuff re-imagined as `dsc_script`. It currently inherits from `Script`, though I'm starting to think a compromise here is to just make it `Script`-like, rather than to inherit, as some attributes, e.g. `creates`, are somewhat meaningless (the resource is essentially idempotent without the need for `creates` or even guards). Also, while it might be cool in some ways, I don't actually want it to be used as a guard, and that's possible if it inherits from `Script`.