Bump yargs-parser and mocha in /challenges/planning

Bumps [yargs-parser](https://github.com/yargs/yargs-parser) to 13.1.2 and updates ancestor dependency [mocha](https://github.com/mochajs/mocha). These dependencies need to be updated together.


Updates `yargs-parser` from 11.1.1 to 13.1.2
- [Release notes](https://github.com/yargs/yargs-parser/releases)
- [Changelog](https://github.com/yargs/yargs-parser/blob/main/docs/CHANGELOG-full.md)
- [Commits](https://github.com/yargs/yargs-parser/commits)

Updates `mocha` from 6.2.0 to 6.2.3
- [Release notes](https://github.com/mochajs/mocha/releases)
- [Changelog](https://github.com/mochajs/mocha/blob/master/CHANGELOG.md)
- [Commits](mochajs/mocha@v6.2.0...v6.2.3)

---
updated-dependencies:
- dependency-name: yargs-parser
  dependency-type: indirect
- dependency-name: mocha
  dependency-type: direct:production
...

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Author: dependabot[bot]