Add a `to_json` method to top-level namespace

[Komnomnomnom]

To explicitly encourage / support serialisation of other objects (e.g. numpy arrays) and better support custom JSON serialisation of Pandas types. As discussed in #9130.

Would welcome more thoughts / ideas on this. Not 100% sold myself.

[cpcloud]

the reason this is nice is because i don't want to be embed dataframe json strings in other json, i want to have proper json with my dataframe and read it too :)

[WillAyd]

This is almost 5 years late but for now I think this is out of scope for pandas; probably would be a separate JSON library if anything