Add json parser reporting accurate source locations.

The json parser is a copy of "package:json/json.dart" where the parser has been modified to report accurate source locations (in absolute position from the beginning of the contents) to the [JsonListener] interface.

The parser will be used to implement "smarter" validators for json file with a specific format (e.g. manifest.json).
Yes, I was hoping to keep this library at a low level. Also, there is a follow up Pull Request coming that creates a "unittest" top level app to run tests for this library as a Dart command line app.

So, we are ok leaving these definitions here, right?