refactor(v2): i18n cleanups / refactors (#4405)

* chore: fix various intl stuff
- remove intl-locales-supported & intl since they're deprecated and only
needed for IE11
- add new polyfills for node 12
- clean up babel intl extractor
- reset jest test timezone to UTC so it passes even for East Coast
contributor

* chore: change build to include Node 14

* docs: update i18n reqs

packages/docusaurus-theme-common/src/utils/usePluralForm.ts

@@ -65,7 +65,8 @@ function useLocalePluralForms(): LocalePluralForms {
     i18n: {currentLocale},
   } = useDocusaurusContext();
   return useMemo(() => {
-    if (Intl && Intl.PluralRules) {
+    // @ts-expect-error checking Intl.PluralRules in case browser doesn't have it (e.g Safari 12-)
+    if (Intl.PluralRules) {
       try {
         return createLocalePluralForms(currentLocale);
       } catch (e) {