Reverse dependency checks

Performs set up for checking the reverse dependencies of an R package, as implemented by the revdepcheck package:

• Adds revdep directory and adds it to .Rbuildignore

• Populates revdep/.gitignore to prevent tracking of various revdep artefacts

• Creates revdep/email.yml for use with revdepcheck::revdep_email()

• Prompts user to run the checks with revdepcheck::revdep_check()

use_revdep()