Implement `navigator.userAgentData` as a built-in API

[jimmywarting]

I’d like to suggest implementing navigator.userAgentData as a built-in global API.

This would help bring better support for web-standard APIs and improve feature parity between browser and runtime environments. It's especially useful in universal (isomorphic) codebases where one shouldn't have to rely on environment-specific globals like process, or check for the presence of Deno or Bun in globalThis.

Having navigator.userAgentData built in would provide a clean and consistent way to detect key runtime details such as:

• the brand (e.g. Deno, Bun, Node)
• the version of the runtime
• the platform (e.g. macOS, Windows, Linux, etc.)

This would be highly useful in scenarios where server-side code needs to branch based on the environment but still adhere to web platform standards. It removes the need for runtime-specific APIs like process.version or string parsing from user agent headers.

Please consider supporting this as a native part of the runtime.

[saghul]

Makes sense! If you're up for it a PR would be welcome!