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/filesystem.js

@@ -57,7 +57,7 @@ var FILE_BUFFER_LENGTH = 64 * 1024,
 exports.copyFile = function(srcPath,dstPath) {
 	// Create buffer if required
 	if(!fileBuffer) {
-		fileBuffer = new Buffer(FILE_BUFFER_LENGTH);
+		fileBuffer = Buffer.alloc(FILE_BUFFER_LENGTH);
 	}
 	// Create any directories in the destination
 	$tw.utils.createDirectory(path.dirname(dstPath));

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();
 	}
 };