Check for Full Site Editing on Jetpack sites

* check if FSE is active
* also check if the theme is supported

sal/class.json-api-site-jetpack.php

@@ -187,6 +187,17 @@ function current_user_can( $role ) {
 		return current_user_can( $role );
 	}
 
+	function is_fse_active() {
+		$fse_enabled = is_plugin_active( 'full-site-editing/full-site-editing-plugin.php' );
+		$has_method = method_exists( '\A8C\FSE\Full_Site_Editing', 'is_supported_theme' );
+		if ( $fse_enabled && $has_method ) {
+			$fse = \A8C\FSE\Full_Site_Editing::get_instance();
+			$slug = get_option( 'stylesheet' );
+			return $fse->is_supported_theme( $slug );
+		}
+		return false;
+	}
+
 	/**
 	 * Post functions
 	 */