✨(react) add a nano button, proper icon sizes

Previously we had to hack the button in order to display small clickable
icons, like the clear button in the DatePicker. By introducing a new
nano button we can natively achieve that. Also make more homegenous
font size and icon sizes in existing button sizes.

.changeset/heavy-scissors-sing.md

@@ -0,0 +1,5 @@
+---
+"@openfun/cunningham-react": minor
+---
+
+add Button nano size