Bump babel-plugin-istanbul from 4.1.6 to 6.1.1

Bumps [babel-plugin-istanbul](https://github.com/istanbuljs/babel-plugin-istanbul) from 4.1.6 to 6.1.1.
- [Release notes](https://github.com/istanbuljs/babel-plugin-istanbul/releases)
- [Changelog](https://github.com/istanbuljs/babel-plugin-istanbul/blob/master/CHANGELOG.md)
- [Commits](istanbuljs/babel-plugin-istanbul@v4.1.6...v6.1.1)

---
updated-dependencies:
- dependency-name: babel-plugin-istanbul
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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