Add DSL for after_install and similar blocks

Refs #5268

This is just a prototype version. Some tests are broken and others missing, but I wanted to get feedback on the general direction and style advice so that I can continue working.
Turns out `suppressMoveToApplications` is not a standard at all, and most of the Casks use `moveToApplicationsFolderAlertSuppress` instead. There is also `kNTMoveToApplicationsFolderAlertSuppress`.

We may also need a generic DSL method for accessing `defaults`.