🧑‍💻(tokens) add getThemesFromGlobals utils

Export a new util method `getThemesFromGlobals` to easily generate theme with
its variant only by providing a partial globals object. By default it returns
both available theme variants (light & dark). Through options you can prefix
variant property keys, only generate theme with a subset of variant and also
overrides/extend theme.

apps/demo/src/Onboarding.tsx

@@ -1,5 +1,10 @@
 import React from "react";
-import { Button, Modal, ModalSize, useModal } from "@gouvfr-lasuite/cunningham-react";
+import {
+  Button,
+  Modal,
+  ModalSize,
+  useModal,
+} from "@gouvfr-lasuite/cunningham-react";
 
 const Onboarding = () => {
   const modal = useModal({