Add GrammarChecker

[itszhanghere]

Pull Request Template

Please ensure all items below are checked before creating a pull request:

• My changes were made in the resources folder, not in the auto-generated README.md file or db folder
• The resource is highly or exclusively related to programming and development
• My submission is formatted according to the guidelines in the contributing guide
• My submission is ordered alphabetically based on the resource name
• My submission has a useful description
• I have searched the repository for any relevant issues or pull requests

---

Summary by cubic

Added GrammarChecker to the resources list to expand writing tools and improve discoverability for grammar correction.

• New Features
  • New resource entry with name, description, categories (Writing, AI, Productivity), URL, and keywords.

^{Written for commit 1950534. Summary will update automatically on new commits.}

[github-actions]

New resource link: https://aigrammarchecker.io/

[cubic-dev-ai]

No issues found across 1 file