refactor: convert to TypeScript with Biome linting
- Convert index.js to src/index.ts with full type definitions
- Add tsconfig.json with strict TypeScript config
- Add biome.json for linting and formatting
- Update package.json with ESM, build scripts, and devDependencies
- Add .env.example template for easy setup
- Add src/types.d.ts for untyped npm packages
- Update README with TypeScript commands and workflow
BREAKING: Now requires 'npm run build' before production use