[Dank Battery Alerts] Add option to shutdown device on low battery

Basically the title, for example I have alerts at 20 and critical alerts at 10 percent, and it would be cool to have an option to automatically shutdown or hibernate laptop at 2%. It possibly could prevent filesystem corruption or other bad stuff happening when battery dies.

The issue names no files, tests, or entry points. Start by locating the existing battery-alert implementation and its settings entry point, then trace how low-battery thresholds trigger alerts. Done means users can choose an automatic shutdown or hibernate action at the requested low-battery level.

Written by the indexing model from the issue text.