Add mgos_invoke_cb() option to use xQueueSendToFront...()

[Harvie]

I need to use relatively complex code in ESP32 GPIO ISR (to handle complex protocol in time).
But it turned out, i can't just tag everything IRAM_ATTR... It panics and i don't know why.

What is working for me is using mgos_invoke_cb to switch from ISR context to main context, where i can use complex libraries... But i feel like it's slow and other events and timers can mess me up. Is there way around this?

mgos_invoke_cb() currently uses xQueueSendToBackFromISR(). What if we used xQueueSendToFrontFromISR() instead? That way i probably can get more priority over other stuff to be able to handle my communication on time... And get executed ASAP after return from ISR...

https://www.freertos.org/xQueueSendToFront.html

IRAM bool mgos_invoke_cb(mgos_cb_t cb, void *arg, bool from_isr) {
  struct mgos_event e = {.cb = cb, .arg = arg};
  if (from_isr) {
    BaseType_t should_yield = false;
    if (!xQueueSendToBackFromISR(s_main_queue, &e, &should_yield)) {
      return false;
    }
    YIELD_FROM_ISR(should_yield);
    return true;
  } else {
    return xQueueSendToBack(s_main_queue, &e, 10);
  }
}

[Harvie]

Moved here: cesanta/mongoose-os#537