General: Escape nonce used for block theme activation.

Add `wp_json_encode()` as an escaping function when setting the `WP_BLOCK_THEME_ACTIVATE_NONCE` global on the block theme preview screen.

This account for custom nonce implementations making use of special characters that require escaping in JavaScript strings.

Props antonvlasenko, ramonopoly.
Fixes #58712.



git-svn-id: https://develop.svn.wordpress.org/trunk@56218 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme-previews.php

@@ -70,7 +70,7 @@ function wp_block_theme_activate_nonce() {
 	$nonce_handle = 'switch-theme_' . wp_get_theme_preview_path();
 	?>
 	<script type="text/javascript">
-		window.WP_BLOCK_THEME_ACTIVATE_NONCE = '<?php echo wp_create_nonce( $nonce_handle ); ?>';
+		window.WP_BLOCK_THEME_ACTIVATE_NONCE = <?php echo wp_json_encode( wp_create_nonce( $nonce_handle ) ); ?>;
 	</script>
 	<?php
 }