✨(react) add multi select

Adding this new variant makes necessary to reorganize the files to keep
a clear separations of concerns. As of now Select/index.tsx is just an
entrypoint to render either the mono or multi variant of the select.

.changeset/seven-bulldogs-bow.md

@@ -0,0 +1,5 @@
+---
+"@openfun/cunningham-react": minor
+---
+
+add multi select