Editor: Introduce sticky position block support.

In the Gutenberg plugin, a position block support feature was introduced last year, that allows a Group block to be set to a "sticky" position, meaning that when the page scrolls, the block will stick to the top of the window.

This change merges the "sticky" position feature for blocks introduced in Gutenberg 15.0.

Props andrewserong, flixos90, mukesh27.
Fixes #57618.

git-svn-id: https://develop.svn.wordpress.org/trunk@55285 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/theme/wpThemeJson.php

@@ -274,6 +274,9 @@ class Tests_Theme_wpThemeJson extends WP_UnitTestCase {
 			'dimensions' => array(
 				'minHeight' => true,
 			),
+			'position'   => array(
+				'sticky' => true,
+			),
 			'spacing'    => array(
 				'blockGap' => false,
 				'margin'   => true,
@@ -301,6 +304,9 @@ class Tests_Theme_wpThemeJson extends WP_UnitTestCase {
 					'dimensions' => array(
 						'minHeight' => true,
 					),
+					'position'   => array(
+						'sticky' => true,
+					),
 					'spacing'    => array(
 						'blockGap' => false,
 						'margin'   => true,