nice!view: battery percentage

[schmidma]

Add text in the battery symbol to show battery percentage. Can be enabled by setting CONFIG_NICE_VIEW_WIDGET_BATTERY_SHOW_PERCENTAGE=y.

PR check-list

• [x] Branch has a clean commit history
• ~~[ ] Additional tests are included, if changing behaviors/core code that is testable.~~
• [x] Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• [x] Pre-commit used to check formatting of files, commit messages, etc.
• [x] Includes any necessary documentation changes.