Model100: Use the real Serial, instead of the default NoSerial

By default, `kaleidoscope::device::Base<>` uses a dummy serial implementation,
and devices must override that to use the proper one. For the Model01, on which
the Model100 plugin was based on, this is done by the `ATMega32U4Keyboard`
class. For the Model100, we need to do it ourselves, because we're deriving from
Base directly.

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

plugins/Kaleidoscope-Hardware-Keyboardio-Model100/src/kaleidoscope/device/keyboardio/Model100.h

@@ -138,6 +138,10 @@ class Model100 : public kaleidoscope::device::Base<Model100Props> {
  public:
   void setup();
 
+  auto serialPort() -> decltype(Serial) & {
+    return Serial;
+  }
+
   static void enableHardwareTestMode();
 };