Nuxt 3 migration preparation: remove some properties of env from nuxt config
as Nuxt 3 has not a possibility of passing configuring env, this MR aims to remove those which can be inlined / replaced with another mechanism. it actually also improves type safety, also removes some global state from classes.ts which gets us closer to single instance (there are still references to data tho). other instances might later end up in runtimeConfig or via another mechansim – but before I change things that might not be necessary, I left it at an incomplete stance.
(this branch is based on 385-multiple-example-sentences-per-morpheme-category, so the diff contains changes from the other branch too)