encoding/json: implement Is on SyntaxError

Allows users to check:

      errors.Is(err, &json.SyntaxError{})

which is the recommended way of checking for kinds of errors.

Change-Id: I20dc805f20212765e9936a82d9cb7822e73ec4ef
GitHub-Last-Rev: e2627ccf8e2a00cc3459bb9fee86c3c8675a33af
GitHub-Pull-Request: golang/go#41210
Reviewed-on: https://go-review.googlesource.com/c/go/+/253037


Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>