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)

src/WebSockets.cpp

@@ -482,7 +482,7 @@ void WebSockets::handleWebsocketPayloadCb(WSclient_t * client, bool ok, uint8_t
         switch(header->opCode) {
             case WSop_text:
                 DEBUG_WEBSOCKETS("[WS][%d][handleWebsocket] text: %s\n", client->num, payload);
-                // no break here!
+                // fallthrough
             case WSop_binary:
             case WSop_continuation:
                 messageReceived(client, header->opCode, payload, header->payloadLen, header->fin);