Add Heroic game launcher

Hi Nonguixers!

Here is a (draft, but ready to go) commit to add the Heroic game launcher: https://heroicgameslauncher.com/

I have tested it works, using the same sandbox home as our Steam package. I can log into Epic Games, download a game, and it runs well. I don't think Heroic needs as many libraries as Steam for the basic operation, but I would guess nearly the same for actually running games. So I just used the Steam inputs but added a couple needed for Heroic (64bit only, and avoids a conflict with grafts currently).

Not tested: nvidia, though I made the packages. Presumably works as well (or not) as Steam currently.

Also: Steam integration won't work right now, although it can create games in Steam, Steam doesn't see Heroic to launch games. Not sure the best way to do this unless you combine both packages? Or use a portal to call out to the host to then run Heroic? Not sure, but I figure that we can fix later.

Please test and let me know what you think! I'll test it again but I think it is good to go for a first round.

Done as one commit, though I suppose the module rename could be split as a separate commit first? Commit message:

nongnu: Add heroic.

• nongnu/packages/steam-client.scm: Rename to ...
• nongnu/packages/game-client.scm: ... this. (heroic-client, heroic-extra-client-libs, heroic-container, heroic-nvidia-container, heroic, heroic-nvidia): New variables.
• nonguix/multi-arch-container.scm (make-container-wrapper): Preserve "^SSL_" for heroic to use curl. (make-internal-script): Add symlink for "/usrshare/glib-2.0".