Added JSON_VALUE for mysql platform

[Hikariii]

fixes #82

[Hikariii]

This means MariaDB will have JSON_VALUE twice

Available is imo the needed addition. It will be available twice. You still have to register every function separately, so they will not conflict if you pick what you need.

[annervisser]

This means MariaDB will have JSON_VALUE twice

Available is imo the needed addition. It will be available twice. You still have to register every function separately, so they will not conflict if you pick what you need.

@Hikariii All other JSON functions in this repo's MySQL directory are also available (and identical) in MariaDB. It seems inconsistent to only have the JsonValue duplicated.

[Hikariii]

@annervisser I just updated json_value to be mysql specific.