Add getColourSupport to interface

Author: Lynesth

src/Terminal.php

@@ -18,6 +18,11 @@ public function getWidth() : int;
      */
     public function getHeight() : int;
 
+    /**
+     * Get the number of colours the terminal supports (1, 8, 256, true colours)
+     */
+    public function getColourSupport() : int;
+
     /**
      * Disables echoing every character back to the terminal. This means
      * we do not have to clear the line when reading.