Add an example to trasmit RS422 signal

[yoneken]

This example modifies the pio_uart_tx sample to implement RS-422 transmit functionality. It utilizes the PIO state machine to generate the differential signal directly, eliminating the need for an external line driver IC to send "Hello, world!"

[lurch]

Example is failing to build in CI:

/opt/actions-runner/_work/pico-examples/pico-examples/pico-examples/pio/rs422_tx/rs422_tx.pio:27.9-12: syntax error, unexpected next
   27 |     jmp next    side 0b01 [3]
      |         ^~~~
/opt/actions-runner/_work/pico-examples/pico-examples/pico-examples/pio/rs422_tx/rs422_tx.pio:30.1-4: syntax error, unexpected next
   30 | next:
      | ^~~~
gmake[2]: *** [pio/rs422_tx/CMakeFiles/pio_rs422_tx_rs422_tx_pio_h.dir/build.make:73: pio/rs422_tx/rs422_tx.pio.h] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:8066: pio/rs422_tx/CMakeFiles/pio_rs422_tx_rs422_tx_pio_h.dir/all] Error 2
gmake[1]: *** Waiting for unfinished jobs....

This is probably because next is a keyword used by the wait and irq PIO instructions, and so pioasm doesn't allow next be used as a label, as identified by the comments on https://github.com/raspberrypi/pico-sdk/issues/1042

[yoneken]

I changed the label. Thank you.

[lurch]

I changed the label. Thank you.

Thanks @yoneken , CI is building cleanly now :slightly_smiling_face: