Upgrade to nuxt-edge

Goal of this MR:

As the latest official Nuxt2 release is from 2021 (see https://github.com/nuxt/nuxt.js/releases), upgrade to nuxt2-edge which at least gets some updates for an undefined period of time (see https://www.npmjs.com/package/nuxt-edge?activeTab=versions).

Closes #287 (closed)

Note: I had to install manually the package ufo, which is required by Nuxt2 but doesn't seem to be in its package.json. Otherwise the build process crashes.

Second note: forget the official roadmap.