🏷️(frontend) fix intl type breakage from dependency

We relied on having a type definition for `Intl.RelativeTimeFormat`
as we believed it existed in the typescript libs. It was really
coming from one of our dependencies extending `Intl` with it.

We can just type assert as the only call that uses `RelativeTimeFormat`
is to assert whether or not to run a polyfill.

src/frontend/index.tsx

@@ -37,8 +37,8 @@ document.addEventListener('DOMContentLoaded', async event => {
     if (!Intl.PluralRules) {
       await import('intl-pluralrules');
     }
-
-    if (!Intl.RelativeTimeFormat) {
+    // TODO: remove type assertion when typescript libs include RelativeTimeFormat
+    if (!(Intl as any).RelativeTimeFormat) {
       await import('@formatjs/intl-relativetimeformat');
       // Get `react-intl`/`formatjs` lang specific parameters and data
       await import(