feat: add lexicon validator tool
- Single-page HTML tool to validate AT Protocol lexicon schemas and records
- NSID dropdown auto-populates from entered lexicons with record types
- Record template auto-generates from required fields with sensible defaults
- Reset button to regenerate template
- Powered by honk library (hexdocs.pm/honk)
- Add prettier config and format.sh for HTML formatting
- Update index.html and README.md