Runs one or more :add steps by name.
Usage
mix starter.add credo
mix starter.add credo,quokkaRun mix starter.add --list to see every available step.
This task runs Starter's built-in steps only. For a package that ships its
own Igniter installer (oban, ash, …), use mix igniter.install — or
{:add, :name} in a starter, which resolves either way.