Blocks: Add context fields to WP_Block_Type

New block context related fields were added as part of https://github.com/WordPress/gutenberg/pull/22686. This changest backports them to WP_Block_Type class.

Props aduth, spacedmonkey, mcsf, epiqueras.
Fixes #47656.  



git-svn-id: https://develop.svn.wordpress.org/trunk@48117 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/admin/includesPost.php

@@ -844,7 +844,7 @@ class Tests_Admin_Includes_Post extends WP_UnitTestCase {
 				'category'    => 'common',
 				'icon'        => 'text',
 				'keywords'    => array(),
-				'supports'    => array(),
+				'usesContext' => array(),
 				'styles'      => array(),
 			),
 			$blocks[ $name ]