Lint Lua using Luacheck

[alerque]

This is dependent on the upstream PR adding builtin support for Pandoc to Luacheck, see https://github.com/lunarmodules/luacheck/pull/115. I'll take it out of draft mode when that is in a release ... which could be sooner rather than later if feedback here/there is reasonable. The main motivation for adding the presets is using it on my own filters ... running it on the Pandoc repo was just a handy way to give it a shaking out.