Deprecate the json_encode and json_decode functions

[soullivaneuh]

Summary

Since PHP 7.3.0, the json_encode and json_decode functions have a JSON_THROW_ON_ERROR to be set in order to throw an exception instead of returning false in case of error.

Also, Rector proposes to automatically adding it trough the official PHP upgrade migration rules.

Is is still interesting to maintain those function inside this project? If not, we may mark them as deprecated. Also, we should update the provided rector migration file accordingly:

safe/rector-migrate.php

Lines 461 to 462 in 3a6e43b

	'json_decode' => 'Safe\json_decode',
	'json_encode' => 'Safe\json_encode',

Any though?