Fabrica Synced Pattern Instances

Adds some basic but vital indexing functionality for Synced Patterns (previously called Reusable Blocks until WP 6.3):

• Shows Patterns in the main WordPress admin menu
• Adds a column to the Patterns list indicating how many times a Synced Pattern appears throughout the site
• This count links to a list of all the content (Posts, Pages, and other public Post Types) which includes the block

Designed to work seamlessly with Fabrica Dashboard: https://wordpress.org/plugins/fabrica-dashboard/