Improve type checking of request data

[joetannenbaum]

All credit to @7nohe from #1537

import { router } from '@inertiajs/vue3'

type RequestData = { message: string }

router.post<RequestData>(url, { title: 'hello' }, options) // <- TypeScript Error
router.post<RequestData>(url, { message: 'hello' }, options) // <- OK