feat: Add `t.has` to check whether a given message exists

[tholander]

import {useTranslations} from 'next-intl';

function AboutPage() {
  const t = useTranslations('AboutPage');

  if (!t.has('title')) {
    notFound()
  }

  // ...
}

Closes #88

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[blazingh]

thanks a lot for this.