WIP: modules: systems: add persist

This is the module that takes care of configuring impermanence at the
system level.

WIP:
    * address FIXMEs
    * activate home-manager persistence?
        * set `programs.fuse.userAllowOther = true;` ?
    * point `age` to persisted paths [1] ?
    * make sure all services and modules are persisted correctly...

[1]: b1d18d25b8

modules/system/default.nix

@@ -9,6 +9,7 @@
     ./language
     ./nix
     ./packages
+    ./persist
     ./podman
     ./users
   ];