borgmatic: Rework borgmatic config to be a little less silly

Instead of globally excluding by pattern, exclude directories that
contain a .borgignore file

This makes the borg config more scalable across different setups

modules/services/borgmatic.nix

@@ -30,14 +30,10 @@
             patterns = [
               #TODO: home.homeDirectory might be usable here
               #TODO: should be user and machine specific
-              "R /home/${username}/Documents"
-              "R /home/${username}/.ssh"
-              "- /home/${username}/Documents/SteamLibrary"
-              "- /home/${username}/Documents/PrismLauncher instances"
-              "- /home/${username}/Documents/.Trash-1000"
-              "- /home/${username}/Documents/VMs/Data"
+              "R /home/${username}"
+              "+ /home/${username}/.ssh"
+              "- /home/${username}/.*"
               "- **/node_modules"
-              "- **/.git"
               "- **/.cache"
               "- **/*build"
               "- **/target"
@@ -48,6 +44,8 @@
               "- **/dist"
               "- **/.pio"
             ];
+
+            extraConfig.exclude_if_present = [ ".borgignore" ];
           };
           retention.keepWeekly = 2;
           storage.extraConfig.ssh_command = "ssh -i /home/${username}/.ssh/id_ed25519_borg";