time: support "," as separator for fractional seconds

Accepts comma "," as a separator for fractional seconds
hence we now accept:
* 2006-01-02 15:04:05,999999999 -0700 MST
* Mon Jan _2 15:04:05,120007 2006
* Mon Jan 2 15:04:05,120007 2006

This change follows the recommendations of ISO 8601 per

   https://en.wikipedia.org/wiki/ISO_8601#cite_note-26

which states

   ISO 8601:2004(E), ISO, 2004-12-01, "4.2.2.4 ...
   the decimal fraction shall be divided from the integer
   part by the decimal sign specified in ISO 31-0, i.e.
   the comma [,] or full stop [.]. Of these, the comma
   is the preferred sign."

Unfortunately, I couldn't directly access the ISO 8601 document
because suddenly it is behind a paywall on the ISO website,
charging CHF 158 (USD 179) for 38 pages :-(

However, this follows publicly available cited literature, as well
as the recommendations from the proposal approval.

Fixes #6189
Updates #27746
Updates #26002
Updates #36145
Updates #43813
Fixes #43823

Change-Id: Ibe96064e8ee27c239be78c880fa561a1a41e190c
Reviewed-on: https://go-review.googlesource.com/c/go/+/300996


Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>