encoding/json: encode \b and \f as '\b' and '\f' in JSON strings

According to RFC 8259, there are exactly 5 control characters
that have a shorter escape sequence than the generic \uXXXX format.

Over the years, we added ad-hoc support for the short sequences:
* https://go.dev/cl/4678046 supports \r and \n
* https://go.dev/cl/162340043 supports \t

This CL completes the set by supporting \b and \f.

This may change the encoding of strings in relatively rare cases,
but is a permissible change since the Go 1 compatibility document does
not guarantee that "json" produces byte-for-byte identical outputs.

In fact, we have made even more observable output changes in the past
such as with https://go.dev/cl/30371 which changes the representation
of many JSON numbers.

This change is to prepare the path forward for a potential
v2 "json" package, which has more consistent encoding of JSON strings.

Change-Id: I11102a0602dfb1a0c14eaad82ed23e8df7553c6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/521675
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Bryan Mills <bcmills@google.com>

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