XIAO SAMD21: Add TX/RX LED On/Off on Arduino IDE menu (like for Seeed Wio Terminal)

[dbstf]

Please add the compiler-flag option to enable/disable the Serial TX/RX LEDs also for the XIAO SAMD21 board, like you already did for the Seeed Wio Terminal by accepting commit c68cc757e623a4b30669e7880a4fb0224bbf6169

Add to boards.txt:
seeed_XIAO_m0.menu.txrxled.on=On
seeed_XIAO_m0.menu.txrxled.off=Off
seeed_XIAO_m0.menu.txrxled.on.build.flags.txrxled=-DTXRXLED_ENABLE

Changes in variants\XIAO_m0\variant.h:
#define PIN_LED2 (12u)
#define PIN_LED3 (11u)
#if defined(TXRXLED_ENABLE)
#define PIN_LED_RXL PIN_LED2
#define PIN_LED_TXL PIN_LED3
#endif
Also, assigning the pin-numbers to PIN_LED2/3 (instead of PIN_LED_RXL/TXL) allows the user to reference the individual LED-pins, while still being able to enable/disable the LEDs with the compiler flag.

Please also correct the copy-paste typo in boards.txt for the Wio Lite MG126
from
# Seeed XIAO M0 (SAMD21)
to
# Wio Lite MG126 (SAMD21)

[Lesords]

Hello,

I'm very sorry to have kept you waiting so long.

I will solve the problem you mentioned later or we also welcome you to submit PR.