Bump flat and nightwatch

Removes [flat](https://github.com/hughsk/flat). It's no longer used after updating ancestor dependency [nightwatch](https://github.com/nightwatchjs/nightwatch). These dependencies need to be updated together.


Removes `flat`

Updates `nightwatch` from 1.3.3 to 1.4.0
- [Release notes](https://github.com/nightwatchjs/nightwatch/releases)
- [Commits](nightwatchjs/nightwatch@v1.3.3...v1.4.0)

---
updated-dependencies:
- dependency-name: flat
  dependency-type: indirect
- dependency-name: nightwatch
  dependency-type: direct:development
...

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