fix: type definitions

[javiertury]

• Allow nullable "data" in BackendModule.read callback. According to the docs, callback(err, null) is valid.
• BackendModule.create is optional.
• BackendModule.readMulti should use MultiReadCallback instead of ReadCallback.

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included <- Not needed (in my opinion)
• documentation is changed or added <- Not needed (in my opinion)

[coveralls]

Coverage remained the same at 93.037% when pulling 9d77723 on javiertury:fix_type_definitions into 8d17fb1 on i18next:master.

[jamuhl]

thank you for contributing - was published in i18next@19.9.0