let micropython handle restarts for REPL mode when an app is running

components/micropython/usermodule/micropython.cmake

components/micropython/usermodule/mp_sys_mode.c

components/micropython/vendor/ports/esp32/main.c

python_payload/mypystubs/sys_mode.pyi

python_payload/st3m/run.py

python_payload/st3m/ui/view.py

sim/fakes/sys_mode.py

sim/fakes/utime.py