refactor: break XDG sesion variables out into a single feature module

hosts/shared/nixos-pc.nix

@@ -119,9 +119,6 @@
       };
     };
 
-    # Completely disable xserver support (RIP X11).
-    xserver.enable = false;
-
     # Enable KMSCON DRM TTY.
     kmscon = {
       enable = true;
@@ -346,6 +343,8 @@
 
   nixpkgs = {
     config.allowUnfree = true;
+
+    # Fix msitools compiling and failing it's test-suite.
     overlays = [
       (final: prev: {
         msitools = prev.msitools.overrideAttrs (old: {
@@ -428,28 +427,7 @@
     ];
 
     shellAliases = import ../../features/aliases.nix { inherit pkgs; };
-
-    sessionVariables = rec {
-      XDG_CACHE_HOME = "$HOME/.cache";
-      XDG_CONFIG_HOME = "$HOME/.config";
-      XDG_DATA_HOME = "$HOME/.local/share";
-      XDG_STATE_HOME = "$HOME/.local/state";
-
-      XDG_DATA_DIRS = [
-        "${pkgs.gsettings-desktop-schemas}/share/gsettings-schemas/${pkgs.gsettings-desktop-schemas.name}"
-        "${pkgs.gtk3}/share/gsettings-schemas/${pkgs.gtk3.name}"
-      ];
-
-      ANDROID_USER_HOME = "$XDG_DATA_HOME/android";
-      AZURE_CONFIG_DIR = "$XDG_DATA_HOME/azure";
-      NPM_CONFIG_INIT_MODULE = "$XDG_CONFIG_HOME/npm/config/npm-init.js";
-      NPM_CONFIG_CACHE = "$XDG_CACHE_HOME/npm";
-      NPM_CONFIG_TMP = "$XDG_RUNTIME_DIR/npm";
-      _JAVA_OPTIONS = "-Djava.util.prefs.userRoot=$XDG_CONFIG_HOME/java";
-      PYTHON_HISTORY = "$XDG_STATE_HOME/python_history";
-      HISTFILE = "$XDG_STATE_HOME/bash/history";
-      GTK2_RC_FILES = "$XDG_CONFIG_HOME/gtk-2.0/gtkrc";
-    };
+    sessionVariables = import ../../features/xdg.nix { inherit pkgs; };
   };
 
   system.stateVersion = "26.05";