Refines a query to match items whose specified property value contains a specified string.
The contains() function refines a PublicPlansQueryBuilder to
only match items where the value of the specified property contains the
specified string. Matching with contains() is case-sensitive, so
"TEXT" does not contain "text".
You can only use contains() with a property whose value is a String.
The property whose value will be compared with the string.
Supported property: slug
The string to look for inside the specified property value.