Add required duplex option to RequestInit

[Benzolio]

see also nodejs/node#46221 and microsoft/TypeScript-DOM-lib-generator#1483

This will prevent the following runtime error:

TypeError: RequestInit: duplex option is required when sending a body.
    at new Request (node:internal/deps/undici/undici:7184:19)
    at createNodeRequest (...node_modules/authey/dist/index.mjs:32:10)

This is the fix as suggested by @KararTY in Issue #4

I can confirm vitest passes, the typecheck does fail, but for the known reason that microsoft/TypeScript-DOM-lib-generator#1483 mentions regarding the incomplete type in their spec which has fallen behind the actual node implementation.

With the single line added for duplex: 'half', the build in dist works without errors when imported into an Express app and used as middleware.

(PR previously submitted as #6)

[wobsoriano]

Thanks!