Editor: Expand Block Bindings for button block.

Add block bindings support for the linkTarget and rel button block attributes.
This allows using custom fiends or pattern overrides for these attributes.

Props glendaviesnz.
Fixes #60481.

git-svn-id: https://develop.svn.wordpress.org/trunk@57576 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-block.php

@@ -241,7 +241,7 @@ private function process_block_bindings() {
 			'core/paragraph' => array( 'content' ),
 			'core/heading'   => array( 'content' ),
 			'core/image'     => array( 'url', 'title', 'alt' ),
-			'core/button'    => array( 'url', 'text' ),
+			'core/button'    => array( 'url', 'text', 'linkTarget', 'rel' ),
 		);
 
 		// If the block doesn't have the bindings property, isn't one of the allowed