Move SetExitKey from core to android, desktop, web

src/rcore_desktop.c

@@ -1512,6 +1512,13 @@ int GetMonitorPhysicalWidth(int monitor)
     return 0;
 }
 
+// Set a custom key to exit program
+// NOTE: default exitKey is ESCAPE
+void SetExitKey(int key)
+{
+    CORE.Input.Keyboard.exitKey = key;
+}
+
 // Get gamepad internal name id
 const char *GetGamepadName(int gamepad)
 {