feat: add registry search response types

[pengyuayn]

Summary

• Add exported TypeScript types for the npm registry /-/v1/search response.
• Model search packages, score details, flags, publisher/maintainer users, trusted publisher metadata, and downloads/dependents fields from the live registry response.
• Add a regression test that fetches a real search response and type-checks it against SearchResponse.

References

Closes #28

Validation

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