Bump handlebars and istanbul

Bumps [handlebars](https://github.com/handlebars-lang/handlebars.js) to 4.7.7 and updates ancestor dependency [istanbul](https://github.com/gotwarlost/istanbul). These dependencies need to be updated together.


Updates `handlebars` from 1.3.0 to 4.7.7
- [Release notes](https://github.com/handlebars-lang/handlebars.js/releases)
- [Changelog](https://github.com/handlebars-lang/handlebars.js/blob/master/release-notes.md)
- [Commits](handlebars-lang/handlebars.js@v1.3.0...v4.7.7)

Updates `istanbul` from 0.2.16 to 0.4.5
- [Changelog](https://github.com/gotwarlost/istanbul/blob/master/CHANGELOG.md)
- [Commits](gotwarlost/istanbul@v0.2.16...v0.4.5)

---
updated-dependencies:
- dependency-name: handlebars
  dependency-type: indirect
- dependency-name: istanbul
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>