Add return types for bg256 and fg256 functions

Author: kodie

src/Output/Color.php

@@ -120,15 +120,15 @@ public function info(string $text, array $style = []): string
     /**
      * Returns the color code for a 256 background color.
      */
-    public function bg256(int $code)
+    public function bg256(int $code): string
     {
         return "48;5;{$code}";
     }
 
     /**
      * Returns the color code for a 256 foreground color.
      */
-    public function fg256(int $code)
+    public function fg256(int $code): string
     {
         return "38;5;{$code}";
     }