Add type-safe support to keyPrefix option

[pedrodurek]

Closes #1387

I've performed some tests and thankfully we won't have a significant increase in compilation time 🎉 .

I'll also update our documentation soon!

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included

Checklist (for documentation change)

• only relevant documentation part is changed (make a diff before you submit the PR)
• motivation/reason is provided

[coveralls]

Coverage remained the same at 96.057% when pulling a145b31 on pedrodurek:support-key-prefix-option into 791957d on i18next:master.

[pedrodurek]

It shouldn't have any breaking changes

[adrai]

I'm not a TypeScript user, so if it looks good for you, it looks good for me ;-)
btw: keyPrefix is an i18next core feature (https://www.i18next.com/overview/api#getfixedt), so maybe it also needs some types updates in i18next?
do you want me to merge?

[pedrodurek]

Hey @adrai, until we move the type-safe feature to the i18next lib, we can safely merge this one

[adrai]

Should we wait to release a new version until #1367 gets done?

[pedrodurek]

Actually, #1367 is good to go

[adrai]

included in v11.13.0