BREAKING - "i2s.h" removed, I2S library added (#8108)

* BREAKING - "i2s.h" removed, I2S library added

An "i2S.h" header was present in prior cores, but this conflicts with the
Arduino standard I2S.h header for the I2S class under Windows (because of
case insensitivity).

Initial 3.0.0 release has a redirect "i2s.h" file in the cores directory to
redirect to the "I2S.h" file in the library, but under Windows this resulted
in the IDE not building the I2S library and link errors.

Remove the offending header.  Code will need to mode to including "I2S.h"
instead (which will include both the Arduino class as well as the low-level
definitions).

Fixes #8107

* Update examples to use proper I2S.h version

cores/esp8266/i2s.h

@@ -1,12 +0,0 @@
-// This include file is a hack to ensure backward compatibility with
-// pre 3.0.0 versions of the core.  There was a *lowercase* "i2s.h"
-// header which was in this directory, now renamed to "core_esp82i66s.h"
-// But, the I2S class has a header, "I2S.h" in uppercase.  On Linux
-// the two names are different, but on Windows it's case-insensitive
-// so the names conflict.
-//
-// Avoid the issue by preserving the old i2s.h file and have it redirect
-// to I2S.h which will give the ESP8266-specific functions as well as
-// the generic I2S class.
-
-#include "../../libraries/I2S/src/I2S.h"

libraries/esp8266/examples/I2SInput/I2SInput.ino

@@ -28,7 +28,7 @@
 */
 
 #include <ESP8266WiFi.h>
-#include <i2s.h>
+#include <I2S.h>
 
 void setup() {
   Serial.begin(115200);

libraries/esp8266/examples/I2STransmit/I2STransmit.ino

@@ -10,7 +10,7 @@
 
 #include <ESP8266WiFi.h>
 #include <WiFiUdp.h>
-#include <i2s.h>
+#include <I2S.h>
 
 #ifndef STASSID
 #define STASSID "your-ssid"