Allow using `hs.json.encode` on non-table types

`hs.json.encode` can currently only encode tables - it would be nice to have this extended to support all other JSON types - booleans, numbers, strings, null. 

`NSJSONSerialization` can write non-array/non-dictionary objects with `NSJSONWritingFragmentsAllowed`: https://developer.apple.com/documentation/foundation/nsjsonwritingoptions/nsjsonwritingfragmentsallowed