I thought the TextInput would be an easy component to start with. I was
wrong, but hey, at least this mostly works. I also left some TODOs in
the code as a treat.

Bug: T369505

Use them as CSS variables, so they’ll automatically be updated when
MediaWiki ships a new Codex version without having to rebuild the app
(and they can depend on the skin too). To avoid including all the design
tokens in the build anyway (Vite doesn’t really support excluding CSS
imports yet, see vitejs/vite#18037), we have
to make sure to not import the tokens anywhere in the app (i.e. neither
in NewLexemeForm.vue nor in App.vue) – only directly in the index.html
(dev entry point).

Bug: T369505