hardware: Introduce KeyboardHardware.has_leds

Intended to make it easier for LED-using code to bail out early, this simple
constant will tell us whether the board has LEDs or if it does not.

Because this will be used regularly, by code not close to the hardware,
`Kaleidoscope.has_leds` is an alias to `KeyboardHardware.has_leds`.

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

src/kaleidoscope/Kaleidoscope.h

@@ -97,6 +97,8 @@ class Kaleidoscope_ {
   void setup(void);
   void loop(void);
 
+  static constexpr bool has_leds = (KeyboardHardware.led_count > 0);
+
   /** Detaching from / attaching to the host.
    *
    * These two functions wrap the hardware plugin's similarly named functions.