Add subcommand to run (or sugggest how to run) LuaRocks with a matching VM version

[alerque]

Already in v0.14.14 we added some suggestions to \use that show how to run LuaRocks with the right Lua version to match SILE, but we might be able to go one step farther and provide a subcommand that wraps LuaRocks entirely and invokes it with the right arguments behind the scenes. If not invoking it directly, at least spitting out a suggested command might work.