Extract pkg cmd abstractions and clean up all migrations (#1291)

* No using custom chromium.desktop any more

* Spacing

* Rely on $OMARCHY_PATH

* Add common pkg and cmd functions for use in migrations and elsewhere

* Simple pkg-adds

* Later migration does it

* Ensure running migrations on older installs have the new pkg/cmds available

* Spacing

* Use new abstractions

* Installed in later migration

* Needless comment

* Use new commands

* Fix package name from 'batt' to 'bat'

migrations/1752292967.sh

@@ -1,5 +1,6 @@
 echo "Update to use UWSM and seamless login"
-if ! command -v uwsm &>/dev/null; then
+
+if cmd-missing uwsm; then
   sudo rm -f /etc/systemd/system/getty@tty1.service.d/override.conf
   sudo rmdir /etc/systemd/system/getty@tty1.service.d/ 2>/dev/null || true
 
@@ -14,5 +15,5 @@ if ! command -v uwsm &>/dev/null; then
     sed -i 's/^GTK_IM_MODULE=fcitx$//' "$HOME/.config/environment.d/fcitx.conf"
   fi
 
-  source ~/.local/share/omarchy/install/login/plymouth.sh
+  source $OMARCHY_PATH/install/login/plymouth.sh
 fi