Bump underscore and @stryker-mutator/core

Bumps [underscore](https://github.com/jashkenas/underscore) to 1.13.6 and updates ancestor dependency [@stryker-mutator/core](https://github.com/stryker-mutator/stryker-js/tree/HEAD/packages/core). These dependencies need to be updated together.


Updates `underscore` from 1.8.3 to 1.13.6
- [Release notes](https://github.com/jashkenas/underscore/releases)
- [Commits](jashkenas/underscore@1.8.3...1.13.6)

Updates `@stryker-mutator/core` from 3.3.1 to 6.3.0
- [Release notes](https://github.com/stryker-mutator/stryker-js/releases)
- [Changelog](https://github.com/stryker-mutator/stryker-js/blob/master/packages/core/CHANGELOG.md)
- [Commits](https://github.com/stryker-mutator/stryker-js/commits/v6.3.0/packages/core)

---
updated-dependencies:
- dependency-name: underscore
  dependency-type: indirect
- dependency-name: "@stryker-mutator/core"
  dependency-type: direct:development
...

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