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)

Author: earlephilhower

README.md

@@ -26,6 +26,7 @@ a WebSocket Server and Client for Arduino based on RFC6455.
  - ESP8266 [Arduino for ESP8266](https://github.com/esp8266/Arduino/)
  - ESP32 [Arduino for ESP32](https://github.com/espressif/arduino-esp32)
  - ESP31B
+ - Raspberry Pi Pico W [Arduino for Pico](https://github.com/earlephilhower/arduino-pico)
  - Particle with STM32 ARM Cortex M3
  - ATmega328 with Ethernet Shield (ATmega branch)
  - ATmega328 with enc28j60 (ATmega branch)

examples/esp8266/WebSocketClient/WebSocketClient.ino renamed to examples/esp8266_pico/WebSocketClient/WebSocketClient.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>