Major UART communication improvement

- the UART communication is improved based on UART Idle line detection interrupt
- both Tx and Rx are efficiently handled using DMA

Other:
- minor visual improvements

Inc/gd32f1x0_it.h

@@ -37,6 +37,8 @@ void DebugMon_Handler(void);
 void PendSV_Handler(void);
 /* SysTick handle function */
 void SysTick_Handler(void);
+/* USART0 handle function */
+void USART1_IRQHandler(void);
 /* I2C0 event handle function */
 void I2C0_EV_IRQHandler(void);
 /* I2C0 error handle function */