Change format according to current battery level

Icon fonts like provide different glyphs for different battery levels

E.g. [Font Awesome](https://forkaweso.me/Fork-Awesome/icons/) has 5 levels

> 

Currently I see no method of changing the output format based on what the current battery level is. Perhaps this could be implemented in a similar way to threshold levels?

Other module where this can be relevant for similar reasons is volume.

Start by inspecting the existing battery module and its threshold-level handling, then compare the volume module for similar output behavior. Define how format selection should respond to battery levels and what compatibility expectations apply; the work is done when the behavior is implemented and covered for the relevant levels.

Written by the indexing model from the issue text.