allow fallbackLng as function

[markdon]

Enables a function to be specified for fallbackLng. Discussed as a solution to #1506. This function can return either of the other existing types: string, array or object.

I felt like I was adding a lot of code to test this small feature, but a smarter way to test this hasn't come to me yet.

Checklist says this will want documentation 🤔 Coming soon I think.

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included
• documentation is changed or added function for fallbackLng i18next-gitbook#140

[coveralls]

Coverage increased (+0.007%) to 92.982% when pulling 9f53732 on markdon:master into ee7d49d on i18next:master.

[adrai]

Can you extend the types? https://github.com/i18next/i18next/blob/master/index.d.ts#L5

[markdon]

Documentation on i18next/i18next-gitbook#140

[adrai]

lgtm

[jamuhl]

thank you for the complete PR (even including docs 👍)

was published in i18next@19.8.0