[beman-tidy] Implement cmake.no_single_use_vars

[neatudarius]

cmake.no_single_use_vars from beman_standard.md

• Implement check() (dry-run mode)
• Implement fix (fix-inplace mode)

Docs:

• infra@tools/beman-tidy/docs/dev-guide.md - Adding a new check
• infra@tools/beman-tidy/docs/dev-guide.md - Testing
• infra@tools/beman-tidy/README.md - Public usage API