getFixedExists should support function getFixedT

[gerwinbrunner]

🚀 Feature Proposal

I think it would be good to have a getFixedExists function similar to the getFixedT

Motivation

When you do translations on the server side - i.e. emails that are language-specific - then you want to explicitly set a language.
For that the getFixedT function comes in handy.

It would also be great in this scenario to have the same language restricted getFixedExists function.

Example

const t = getFixedT('de');
const exists = getFixedExists('de');

if (exists(email.feature.text)) {
const text = t('email.feature.text')
// do more useful stuff here
}

[adrai]

feel free to provide a PR with tests