Removed "label" properties which no longer have any function.
Labels are still used as layer names and as identifiers for sending
behaviors between sides of a split keyboard, so those have been left
alone for now.
Updated existing pre-commit hooks and added some new hooks:
- Remove trailing whitespace
- Ensure every non-empty file ends with a new line
- Check YAML file validity
- Prevent adding large files
- Ensure any scripts with shebangs are executable
Added a GitHub action to run pre-commit on every commit. Removed any
existing actions which duplicate pre-commit.
Ran pre-commit on the codebase.
