HTTP Vary Header Manipulator

`vary` is a focused Node.js utility library designed for manipulating the HTTP `Vary` response header. It provides a simple API to append fields to the `Vary` header of a Node.js `http.ServerResponse` object or directly to an existing `Vary` header string. Currently at version 1.1.2, the package maintains a stable and slow release cadence, primarily focusing on performance improvements and input validation rather than frequent feature additions. Its key differentiator is its singular purpose, offering a reliable and minimal solution for ensuring correct cache behavior when content varies based on request headers, without introducing additional HTTP framework overhead. It is a foundational utility used by many HTTP servers and middleware in the Node.js ecosystem.