support reuse of keys from front-matter

[Dexus]

I have some keys, and I would love to have the possibility to reuse them like:

---
title: "Projekte"
translationKey: "page-projects"
eleventyNavigation:
  key: "{{ translationKey }}"
  title: "{{ title }}"
---

[nford]

https://www.11ty.dev/docs/data-computed/#using-a-template-string