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).
It does not have too, but as part of testing the source locations returned by the parser, it is nice to add "non standard" spacing to ensure the test covers corner cases.