[Security] Bump handlebars from 4.7.6 to 4.7.7 (#241)

Bumps [handlebars](https://github.com/wycats/handlebars.js) from 4.7.6 to 4.7.7. **This update includes a security fix.**
- [Release notes](https://github.com/wycats/handlebars.js/releases)
- [Changelog](https://github.com/handlebars-lang/handlebars.js/blob/master/release-notes.md)
- [Commits](handlebars-lang/handlebars.js@v4.7.6...v4.7.7)

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