Script Loader: Add function_exists() checks for is_admin() and current_theme_supports(), to accomodate for using WP_Dependencies as a standalone class.

Remove `<![CDATA[` when outputting HTML5 script tags. 

Props azaozz.
Fixes #42804.
Built from https://develop.svn.wordpress.org/trunk@46287


git-svn-id: http://core.svn.wordpress.org/trunk@46099 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class.wp-styles.php

@@ -117,7 +117,11 @@ class WP_Styles extends WP_Dependencies {
 	 * @since 2.6.0
 	 */
 	public function __construct() {
-		if ( ! is_admin() && ! current_theme_supports( 'html5', 'style' ) ) {
+		if (
+			function_exists( 'is_admin' ) && ! is_admin()
+		&&
+			function_exists( 'current_theme_supports' ) && ! current_theme_supports( 'html5', 'style' )
+		) {
 			$this->type_attr = " type='text/css'";
 		}