Add "date_time" category annotation for SmartDateItem

Problem/Motivation

Part of the changes implemented in ✨ Make field selection less overwhelming by introducing groups Fixed was a revised category annotation within the FieldType. Smart Date should set this to "date_time" to align with core fields.

Proposed resolution

Add this to the annotation. Note that additional changes to the category set have been proposed, for example 📌 Consider replacing hook_field_type_category_info with YML based plugin discovery Fixed .