Don't use the undici shim for Node

[vixalien]

When compiling for Node.js, the package uses undici to shim for Request/Response, etc.. however, this causes issues in production, when using code that checks for response instanceof Response or similar.

Because the response got is not an actual Response, it causes issues. I think undici should be dropped since the fetch API is supported in Node.js 18+

[kitsonk]

Since oak migrated to JSR, it no longer uses undici and we have stopped publishing the npm package.

You can use more recent versions of oak under Node.js via the package published on JSR like:

npx jsr add @oak/oak