Block Editor: Add theme support flag to opt-out of Core Block Patterns.

WordPress Core comes by default with a number of block patterns and a frequent request was to be able to opt-out of the Core block patterns. 
You can now opt-out using remove_theme_support( 'core-block-patterns' )

Props desrosj, nosolosw.
Fixes #50669.

Built from https://develop.svn.wordpress.org/trunk@48492


git-svn-id: https://core.svn.wordpress.org/trunk@48254 1a063a9b-81f0-0310-95a4-ce76da25c4cd