feat: add stringify

Author: glennsl

Diff for: src/Json.res

@@ -15,3 +15,5 @@ let parseExn = str =>
   | Js.Exn.Error(ex) =>
     raise(ParseError(ex->Js.Exn.message->Js.Option.getWithDefault("Unknown error", _)))
   }
+
+@val external stringify: Js.Json.t => string = "JSON.stringify"

Diff for: src/Json.resi

@@ -7,3 +7,5 @@ let decode: (Js.Json.t, Decode.t<'a>) => result<'a, string>
 
 let parse: string => result<Js.Json.t, string>
 let parseExn: string => Js.Json.t
+
+@val external stringify: Js.Json.t => string = "JSON.stringify"