Add named constants for the 3rd, 4th, and 13th most popular formats.
Fixes#52746
Change-Id: I7ce92e44dcae18c089124f1d6f5bc2d6359d436c
Reviewed-on: https://go-review.googlesource.com/c/go/+/412495
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
