Extend lint to reject 'blank' files

[zvecr]

Description

This will cover the legacy rules.mk files containing

# This file intentionally left blank

As well as the older template config.h files that only contained comments.

Code is mostly set up to also cover keymaps, but this will be enabled separately due to some files maybe having "useful" comments.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).