Add "connect UART" option

[mithro]

The KianV uLinux SoC needs the following code to allow talking to the UART (found in this google doc);

import uselect
import sys
from machine import UART


uart = UART(0, baudrate=115200, tx=tt.pins.ui_in3.raw_pin, rx=tt.pins.uo_out4.raw_pin)
poll = uselect.poll()
poll.register(sys.stdin, uselect.POLLIN)


while True:
    if poll.poll(0):
        _ = uart.write(sys.stdin.buffer.read(1))
    uart_data = uart.read()
    if uart_data:
        _ = sys.stdout.write(uart_data)

It would be great if there was a button on the commander repl page which automatically connected this up.

Maybe it could even end up as a new "serial port" tab?