♻️(react) migrate Modals to react modal

We encoutered an issue where stacked modal backdrop were not rendered
above the modal below. It was caused by the dialog element that is
natively rendered on the top layer regardless where it is create in
the DOM. So we decided to use react modal that provides hand crafted
dialog, and implementing a11y features.

Closes #314

packages/react/src/components/Provider/index.tsx

@@ -103,7 +103,9 @@ export const CunninghamProvider = ({
   return (
     <CunninghamContext.Provider value={context}>
       <ModalProvider>
-        <ToastProvider>{children}</ToastProvider>
+        <div className="c__app">
+          <ToastProvider>{children}</ToastProvider>
+        </div>
       </ModalProvider>
     </CunninghamContext.Provider>
   );