DEV: Rename I18n imports to discourse-i18n (#23915)

As of #23867 this is now a real package, so updating the imports to
use the real package name, rather than relying on the alias. The
name change in the package name is because `I18n` is not a valid
name as NPM packages must be all lowercase.

This commit also introduces an eslint rule to prevent importing from
the old I18n path.

For themes/plugins, the old 'i18n' name remains functional.

app/assets/javascripts/admin/addon/controllers/admin-user-fields.js

@@ -3,7 +3,7 @@ import { action } from "@ember/object";
 import { gte, sort } from "@ember/object/computed";
 import { inject as service } from "@ember/service";
 import { popupAjaxError } from "discourse/lib/ajax-error";
-import I18n from "I18n";
+import I18n from "discourse-i18n";
 
 const MAX_FIELDS = 30;