Create .gitattributes with lean-package-validator

Follow these steps from the repository root:

1. Run ./vendor/bin/lean-package-validator create to generate a .gitattributes file with all expected export-ignore entries.
2. If you want aligned export-ignore columns, add --align-export-ignores.
3. If you do not want the generated header comment, add --omit-header.
4. If a .lpv file exists, it will be used automatically; otherwise the default PHP preset is used.
5. You can refresh an existing, stale .lpv file with ./vendor/bin/lean-package-validator refresh.