Split Lua setup out from `_main.cfg`.

Split Lua setup out from `_main.cfg` to a new `lua/_main.cfg` file.

To my shame, I had accidentally committed the deletion of the Lua setup
in c5136219009ddaa1a9803b797ebcceaac63da49e, meaning that, until this
commit, which adds `lua/_main.cfg`, master was briefly broken, and it
was my fault.