Add a global JSONX constant just like dart:convert's JSON

[stevenroose]

In fact after this change the global encode() and decode() methods should be made private as well. They can be replaced with JSONX.encode() and JSONX.decode().

[jolleekin]

This is not working as T in JsonxCodec is now dynamic rather than null. I'll change the top level decode to check if type is null or dynamic.

P/S: Following the new coding guidelines, constants should be lowercase.