Port functionality from `jenssegers/agent`

[francislavoie]

We've been using https://github.com/jenssegers/agent for quite a long time, but it seems it's fallen out of maintenance, and hasn't been updated to use Mobile-Detect v3 :frowning_face:

We can't really switch away from it and onto Mobile-Detect though, because Agent adds functionality that's critical to our usecase,

For example, Agent has best-effort OS detection, and a few helper methods like ->isDesktop(), ->browser() and ->platform(). We also chain the output of those methods into ->version() (using the property name returned by those) to get specific browser/OS versions.

Would you be willing to review and port some of that functionality, so we can do away with that library, allowing us to remove a link in the dependency chain?