Add `gh-extension` repository topic for `gh ext search` / `gh ext browse` detection

[andyfeller]

Good afternoon! 👋 I'd like to suggest how more people can discover this GitHub CLI extension by adding the gh-extension topic to the repo.

This allows GitHub CLI users to discover extension repositories in several ways:

• gh-extension topic page
• gh ext search
• gh ext browse

Thank you again for supporting the GitHub CLI extension ecosystem! ❤