Add missed JSON class support

[pinepain]

While we have all that in PHP, it might come in handy under certain circumstances to use V8's JSON::stringify() and JSON::parse().

For those who want to contribute: this task is easy to pick - just draw inspiration from existent php-v8 code style. V8's JSON class is very simple and it has just two static methods, which should be also static in PHP: https://github.com/v8/v8/blob/6441d234c4148e8e34a623ba3e5861a451737ffc/include/v8.h#L1799-L1829

[pinepain]

As of 6.3.117, v8 makes v8::JSON::Stringify take Local<Value> instead of Local<Object> (v8/v8@6716a7f#diff-3dd7a0f46b1e5c1cc67921449e7180ac)