Avoid deprecated new Buffer() usage

See https://alexatnet.com/node-js-10-important-changes/#buffer-1

> Uses of new Buffer() and Buffer() outside of the node_modules directory will now emit a runtime deprecation warning.

More details: https://nodejs.org/api/buffer.html#buffer_buffer_from_buffer_alloc_and_buffer_allocunsafe

core/modules/utils/utils.js

@@ -712,7 +712,7 @@ exports.base64Decode = function(string64) {
 		// TODO
 		throw "$tw.utils.base64Decode() doesn't work in the browser";
 	} else {
-		return (new Buffer(string64,"base64")).toString();
+		return Buffer.from(string64,"base64").toString();
 	}
 };