Adds a new shortcode.
Care should be taken through prefixing or other means to ensure that the shortcode tag being added is unique and will not conflict with other, already-added shortcode tags. In the event of a duplicated tag, the tag loaded last will take precedence.
(string) (Required) Shortcode tag to be searched in post content.
(callable) (Required) The callback function to run when the shortcode is found. Every shortcode callback is passed three parameters by default, including an array of attributes (
$atts), the shortcode content or null if not set (
$content), and finally the shortcode tag itself (
$shortcode_tag), in that order.