This is Jeffrey Friedl's JSON.lua library, copied from http://regex.info/blog/lua/json (source code). It is licensed under [https://creativecommons.org/licenses/by/3.0/deed.en_US CC-BY 3.0].
The suggested loading code of JSON = assert(loadfile "JSON.lua")() will not work in Scribunto. Instead, load the module as follows:
You can then use the module as documented in the module comments.