✨(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.

packages/react/src/components/Forms/Select/tokens.ts

@@ -20,4 +20,6 @@ export const tokens = (defaults: DefaultTokens) => ({
   "background-color": "white",
   "menu-background-color": "white",
   "label-color--focus": defaults.theme.colors["primary-600"],
+  "multi-pill-background-color": defaults.theme.colors["greyscale-200"],
+  "multi-pill-border-radius": "2px",
 });