Update serverSideTranslation args type

[mayone-du]

Thanks for developing a great library!

The useTranslation function completes the namespace defined in CustomTypeOptions on i18next.d.ts, but the serverSideTranslations function does not, so I changed the type.

(If the change causes any inconvenience, close this PR.)

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

[adrai]

Why is that change necessary?
Is there an issue?

[mayone-du]

The reason is that the getServerSideTranslations argument is not type safe.
This is not in the issue, but I created a PR because I was unhappy with it while using it.

[adrai]

I'm not a TS expert, but I don't think Namespace[number] is correct, @pedrodurek @belgattitude?

[mayone-du]

Since the original Namespace type definition is T | T[], Namespace[number] narrows it down to T.

(T contains namespace if it is defined, or a string if not.)

[adrai]

included in v13.2.0