Functions: Let length() function count only Expression or Value

Henceforth the check against HasValueProperty is not required
and could be simplified to check only against valid concrete
implementations. Only Expression and Value can have array values.

Other type will gracefully produce "1" as result, same as before.

Bug: T327082
Change-Id: I812a7e54ad2d9ed955043381a136ddc646cb0b7b

lib/Less/Functions.php

@@ -847,7 +847,7 @@ public function extract( $values, $index ) {
 	}
 
 	public function length( $values ) {
-		$n = ( $values instanceof Less_Tree_HasValueProperty && is_array( $values->value ) ) ?
+		$n = ( $values instanceof Less_Tree_Expression || $values instanceof Less_Tree_Value ) ?
 			count( $values->value ) : 1;
 		return new Less_Tree_Dimension( $n );
 	}