Add the Profile Sync Daemon Package & NixOS Module

Wasn't exactly sure what categories/subdirectories to put these under. It's a daemon that syncs your web browser profiles to tmpfs to increase speed/responsiveness & decrease ssd writes.
If you reboot, the psd.service should be stopped and it's `ExecStop` should be run. That should make all changes persistant.

If your system crashes and the service doesn't get shut down properly, you will lose data up to the last `resync`.