encoding/json: UnmarshalTypeError.Offset and json subparsers

[ibukanov]

This is a followup to #9693 that added UnmarshalTypeError.Offset to record textual position of errors during unmarshaling. The problem with the landed patch is that it does not deal with UnmarshalTypeError returned from implementations of Unmarshaler.UnmarshalJSON(). The offset in such case would reflect an offset from the beginning of the slice, not from the start of the original JSON.

It would be nice to be able to recover the full offset for such cases. For example, UnmarshalTypeError could include a boolean flag indicating that UnmarshalTypeError.Offset should be updated by the caller to reflect the real position in the file. Then the code that handles errors from UnmarshalJSON would do just that.