v3.0.0

cfv v3 integration: syntax + schema + format checking.

- Format checking (warn/strict/off), on by default in warn mode
- SchemaStore enabled by default
- Composite action with pre-built binaries (10-20x faster, macOS support)
- New inputs: format-check, no-config, config
- New output: files-unformatted

See CHANGELOG.md and MIGRATION.md.