feat: add fastly:device module which allows applications to detect a device based on a user-agent

[JakeChampion]

An example application looks like this:

import { Device } from "fastly:device";
addEventListener("fetch", event => {
    event.respondWith(
      Response.json(
        Device.lookup(
          event.request.headers.get('user-agent')
        )
      )
    )
})

[elliottt]

Awesome! 🎉