dkjson: fix JsonState.exception type and make json types more accurate

[goldenstein64]

• Give dkjson.JsonState.exception the same type as dkjson.encodeexception
• Make dkjson.encode and dkjson.JsonState.exception parameter types more accurate
  • Create type InputJsonValue to represent encodable values
  • Create interface InputJsonUserdata to help guard encodable values
• Make dkjson.decode return type more accurate
  • Create type OutputJsonValue to represent possible decoded strings
  • Add type parameter Null for nullval argument

[hishamhm]

This comment is about the userdata, right?

I wonder if it would work to encode this check like this

interface JsonUserdata is userdata
   where getmetatable(self).__tojson
end

type InputJsonValue = string | number | boolean | table | JsonUserdata

this should allow things like

if v is dkjson.JsonUserdata then
   s = dkjson.encode(v)

and

record MyUserdata is dkjson.JsonUserdata
   -- ...
end

(On the phone, can't double check right now if there's any compiler gotchas)

[goldenstein64]

It's for both tables and userdatas. It looks like the JsonUserdata interface is valid with some tweaks, so I'll include it.