✨(tokens) add token references

Previously we were not fully using CSS variables as values used in
CSS were hard-coded one. It wasn't keeping the variable references.
This was causing issues when customizing the theme, because editing
colors was not enough, it was needed to customize also the tokens
using these variables. Now by introducing ref() we can delegate how
to deal with these directly to the generators themselves.

.changeset/chatty-garlics-exercise.md

@@ -0,0 +1,5 @@
+---
+"@openfun/cunningham-tokens": minor
+---
+
+add token references