Bump underscore and grunt-ts

Removes [underscore](https://github.com/jashkenas/underscore). It's no longer used after updating ancestor dependency [grunt-ts](https://github.com/TypeStrong/grunt-ts). These dependencies need to be updated together.


Removes `underscore`

Updates `grunt-ts` from 5.5.1 to 6.0.0-beta.22
- [Release notes](https://github.com/TypeStrong/grunt-ts/releases)
- [Changelog](https://github.com/TypeStrong/grunt-ts/blob/master/CHANGELOG.md)
- [Commits](https://github.com/TypeStrong/grunt-ts/commits)

---
updated-dependencies:
- dependency-name: underscore
  dependency-type: indirect
- dependency-name: grunt-ts
  dependency-type: direct:development
...

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Author: dependabot[bot]