deps: relocate 'c8' to devDependencies

[jayaddison]

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included
• commit message and code follows the Developer's Certification of Origin

The dependency on c8 is used when generating test coverage, and isn't required by the application or CLI at runtime; so I think we can/should move it to the devDependencies category in package.json.

[coveralls]

coverage: 97.26%. remained the same
when pulling fbdfcb6 on openculinary:dependencies/c8-to-devDependencies
into 61241dc on i18next:master.