Port to Raspberry Pi Pico W core

Add support for the onboard WiFi chip on the Raspberry Pi Pico W
(RP2040 based ) board using the arduino-pico Arduino core at
https://github.com/earlephilhower/arduino-pico

The PicoW WiFi stack is a mashup of the ESP8266 and ESP32 cores, so
only minimal changes were required.  Defines a new NETWORK_TYPE for
the PicoW.

ESP8266 examples renames to ESP8266_PICO because they all work unmodified
(except for OTA which is handled differently on the Pico)

examples/esp8266_pico/WebSocketClientOTA/WebSocketClientOTA.ino

@@ -8,15 +8,16 @@
 #include <Arduino.h>
 #include <ArduinoJson.h>
 
-#ifdef ESP8266
+#if defined(ESP8266)
     #include <ESP8266WiFi.h>
     #include <ESP8266mDNS.h>
-	#include <Updater.h>
-#endif
-#ifdef ESP32
+    #include <Updater.h>
+#elif defined(ESP32)
     #include "WiFi.h"
     #include "ESPmDNS.h"
-	#include <Update.h>
+    #include <Update.h>
+#else
+    #error Unsupported device
 #endif
 
 #include <WiFiUdp.h>