Make sure stubs end with newline

Caught this using editorconfig checker:

.castor.stub.php:
        Wrong line endings or no final newline

1 errors found

src/Stub/StubsGenerator.php

@@ -105,7 +105,7 @@ private function generateStubs(string $dest): void
             ],
         ]));
 
-        $code = (new Standard())->prettyPrintFile($stmts);
+        $code = (new Standard())->prettyPrintFile($stmts) . \PHP_EOL;
 
         file_put_contents($dest, $code);
     }