EmmyLua-style function type definitions

[mochaaP]

See https://github.com/orgs/EmmyLua/repositories and https://github.com/LuaLS/lua-language-server/wiki/Annotations.

[mochaaP]

https://gist.github.com/mochaaP/68d9dd2d78bb97e190296a6c27c48629 Just wrote one.

[dsalt]

Maybe convert this into an actual commit or patch? (It'll need an update or two for recent changes.)

[mochaaP]

I don't actually use X11 now, so you're welcomed to modify it :). I attached 0BSD license to the gist.