feat: add modern package json fields

[pengyuayn]

Summary

• Add missing modern package metadata fields to PackageJSON: type, exports, imports, and maintainers.
• Add a regression test covering common conditional exports/imports and maintainer metadata.

References

Closes #107

Validation

• npx tap test/package-json-fields.ts
• npm run eslint -- test/package-json-fields.ts types/index.d.ts
• git diff --check