Deprecate js.Lib.alert().

Closes #2793.

std/js/Lib.hx

@@ -31,8 +31,10 @@ class Lib {
 	}
 
 	/**
-		Display an alert message box containing the given message
+		Display an alert message box containing the given message.
+		@deprecated Use Browser.window.alert() instead.
 	**/
+	@:deprecated("Lib.alert() is deprecated, use Browser.window.alert() instead")
 	public static function alert( v : Dynamic ) {
 		untyped __js__("alert")(js.Boot.__string_rec(v,""));
 	}