flag: add TextVar function

flag: add TextVar function

The TextVar function makes it easier to integrate the flag package
with any Go type that implements encoding.Text{Marshaler,Unmarshaler}.

Fixes #45754

Change-Id: Id23c37d59cf8c9699a7943a22ce27a45eb685c0f
Reviewed-on: https://go-review.googlesource.com/c/go/+/313329
Trust: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>