encoding/json: add Decoder.InputOffset for offset access
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 <8c98250619a5229afaab52d25e022b9e3e1d22a2@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

CAFxX/go,cloudflare/go,akutz/go,akutz/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,akutz/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,akutz/go,cloudflare/go,cloudflare/go,golang/go,akutz/go,golang/go,golang/go,golang/go,golang/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,CAFxX/go,akutz/go,golang/go,cloudflare/go,nawawi/go,akutz/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,akutz/go