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.

plugins/chat/assets/javascripts/lib/discourse-markdown/chat-transcript.js

@@ -1,5 +1,5 @@
 import { performEmojiUnescape } from "pretty-text/emoji";
-import I18n from "I18n";
+import I18n from "discourse-i18n";
 
 let customMarkdownCookFn;