move useI18n to i18n.ts

Since we are using the react-i18n babel loader plugin, useI18n can only
be used once per file. Also we have to remember to add the istanbul
ignore next comment each time we use it. By moving this to a common
file, it can be easily copied and pasted when a new submodule is
created and we don't have to remember the rules when calling it.

src/notifications/NotificationMessage.tsx

@@ -3,9 +3,9 @@
 
 // provides translation for notification text
 
-import { Replacements, useI18n } from '@shopify/react-i18n';
+import { Replacements } from '@shopify/react-i18n';
 import React from 'react';
-import { I18nId } from './i18n';
+import { I18nId, useI18n } from './i18n';
 
 type NotificationMessageProps = {
     messageId: I18nId;
@@ -16,8 +16,7 @@ const NotificationMessage: React.VoidFunctionComponent<NotificationMessageProps>
     messageId,
     replacements,
 }) => {
-    // istanbul ignore next: babel-loader rewrites this line
-    const [i18n] = useI18n();
+    const i18n = useI18n();
 
     let message = i18n.translate(messageId, replacements) as
         | React.ReactElement