Improve developer ergonomics for defining a CustomDetector in TS by making types available

[mrackwitz]

Personally I don't like flying blind, and right now it would be a major hassle to access these types via I18nextBrowserLanguageDetector indirectly. But maybe I'm holding it wrong. As this is only exporting types, this shouldn't be an API breaking change, but it requires Typescript 3.8 from my understanding.

Checklist

• [x] only relevant code is changed (make a diff before you submit the PR)
• [ ] run tests npm run test (I did not, I changed this right here in GitHub)
• [x] tests are included
• [ ] documentation is changed or added

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[adrai]

should already be included in v6.1.3