refactor: port to *Lua* v5.4

[stevenxxiu]

Thanks for the plugin! I ported this to Lua v5.4, so I can use https://github.com/3rd/image.nvim for https://github.com/benlubas/molten-nvim.

I switched to the CFFI library q66/cffi-lua: A portable C FFI for Lua 5.1+.

I tested this via make test. All tests pass.

I haven't tested this on GraphicsMagick, but made the same changes.

Hope you have time to look at this.

[stevenxxiu]

I just realized that Neovim is still on Lua v5.1. So in that case this PR doesn't need to be merged, at least not now.

[leafo]

I won't be able to accept this PR as is, this library is intentionally designed to work in environments with LuaJIT