feat(display): Add label widget

[aggstam]

This is a barebones label widget for displaying a custom message in the center of display.

The widget can be enabled using CONFIG_ZMK_WIDGET_LABEL in the .conf file.
The displayed message is configured using CONFIG_ZMK_WIDGET_LABEL_TEXT(default is "ZMK").
Docs have been updated to include the new configuration options.

Demo example:

My end goal is to create a widget capable of displaying custom icons, using lvgls' image lib.

This PR is a re-open of #2025 after everything was rebased and its ready for review again.

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• 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)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.