Set Breeze as grub theme automatically using /etc/default/grub.d

With recent versions of grub package in Debian, it's now possible to override /etc/default/grub by dropping configuration snippets into /etc/default/grub.d/. This means theme packages like Breeze no longer need to edit the main config file, which belongs to the grub-common package to set the grub theme.

This change adds a file to /etc/default/grub.d/ that sets the Breeze grub theme automatically upon installation. It also adds the d/postinst and d/postrm to run update-grub2, ensuring the configuration is applied and removed properly.

Previously, users had to manually edit /etc/default/grub, which was error-prone, could cause dpkg prompts during upgrades, and modified files belonging to other packages. Now that /etc/default/grub.d/ is supported, we can set the theme cleanly and safely, without conflicting with grub-common.