encoding/json: add Decoder.InputOffset for offset access

This makes Decoder.offset public while renaming it to
Decoder.InputOffset to match encoding/xml Decoder API

Code changes made by Adam Stankiewicz [sheerun@sher.pl]

Fixes #29688

Change-Id: I86dbfd2b2da80160846e92bfa580c53d8d45e2db
Reviewed-on: https://go-review.googlesource.com/c/go/+/200677


Run-TryBot: Johan Brandhorst <johan.brandhorst@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>