Move some MarlinCore and MarlinUI code (#20832)

Marlin/src/feature/twibus.cpp

@@ -28,6 +28,8 @@
 
 #include <Wire.h>
 
+TWIBus i2c;
+
 TWIBus::TWIBus() {
   #if I2C_SLAVE_ADDRESS == 0
     Wire.begin();                  // No address joins the BUS as the master
@@ -155,6 +157,14 @@ void TWIBus::flush() {
     reset();
   }
 
+  void i2c_on_receive(int bytes) { // just echo all bytes received to serial
+    i2c.receive(bytes);
+  }
+
+  void i2c_on_request() {          // just send dummy data for now
+    i2c.reply("Hello World!\n");
+  }
+
 #endif
 
 #if ENABLED(DEBUG_TWIBUS)