Check formatting

[ryanplusplus]

This updates the formatting configuration a bit, applies it to all Lua files, then adds a GitHub Actions workflow to verify that all files are formatted.

I'm not in love with the formatter as it doesn't seem to allow line breaks (or lack of line breaks) to be retained. I disabled it in some specs to improve readability, but was able to keep it enabled in most places.

I'm open to suggestions if anyone knows a better formatter. I usually use the metalua-derived formatter, but it really only handles indentation.

I'm also okay if we want to keep the formatter configuration as a reference but not require the code to match the output of the formatter.

[imolein]

LuaFormatter is not perfect, but the best I found so far in the last years (imo). I used StyLua before, but it has very few configuration options.