Consider named argument flags:JSON_THROW_ON_ERROR for json_ functions as "Safe"

[pyrou]

Since PHP8, named argument allows to pass input data into a function, based on their argument name instead of the argument order.

// PHP 7.3
json_decode($payload, null, 512, JSON_THROW_ON_ERROR);
// PHP 8.0
json_decode($payload, flags: JSON_THROW_ON_ERROR);

Because of this, flags might not be any more always on 4th position. This PR bring support of named argument for json_decode and json_encode JSON_THROW_ON_ERROR flag detection.

[spawnia]

I think this implementation is sound and adds a useful feature, I would love to see it merged and released.

[pyrou]

ping @moufmouf @dbrekelmans :)

[dbrekelmans]

@pyrou if you could resolve the conflict, I'll be happy to merge it