encoding/json: add json/v2 with GOEXPERIMENT=jsonv2 guard

This imports the proposed new v2 JSON API implemented in
github.com/go-json-experiment/json as of commit
d3c622f1b874954c355e60c8e6b6baa5f60d2fed.

When GOEXPERIMENT=jsonv2 is set, the encoding/json/v2 and
encoding/jsontext packages are visible, the encoding/json
package is implemented in terms of encoding/json/v2, and
the encoding/json package include various additional APIs.
(See #71497 for details.)

When GOEXPERIMENT=jsonv2 is not set, the new API is not
present and the encoding/json package is unchanged.

The experimental API is not bound by the Go compatibility
promise and is expected to evolve as updates are made to
the json/v2 proposal.

The contents of encoding/json/internal/jsontest/testdata
are compressed with zstd v1.5.7 with the -19 option.

Fixes #71845
For #71497

Change-Id: Ib8c94e5f0586b6aaa22833190b41cf6ef59f4f01
Reviewed-on: https://go-review.googlesource.com/c/go/+/665796
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/encoding/json/stream_test.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build !goexperiment.jsonv2
+
 package json
 
 import (