POC: Render the user profile tabs in Vue Router

What does this MR do and why?

Refactors the current profile_tabs.vue to a route rendered in a Vue Router. This is the ground work needed so that we can progressively move the individual tab items and move them into the sidebar menu (see #389842).

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

N/A

How to set up and validate locally

1. In rails console enable the profile_tabs_vue feature flag

   Feature.enable(:profile_tabs_vue)

2. Visit a user's profile page (http://localhost:3000/root)
3. There should be no functional difference. Both the sidebar menu and tabs should work normally.

Related to #389842 #497477 (closed)

app/assets/javascripts/pages/users/index.js

 import $ from 'jquery';
 import { setCookie } from '~/lib/utils/common_utils';
 import UserCallout from '~/user_callout';
-import { initProfileTabs, initUserAchievements } from '~/profile';
+import { initUserAchievements, initUserProfileApp } from '~/profile';
 import { initUserActionsApp } from '~/users/profile/actions';
 import UserTabs from './user_tabs';
 
@@ -11,7 +11,7 @@ function initUserProfile(action) {
   // See https://gitlab.com/gitlab-org/gitlab/-/issues/435899.
 
   if (gon.features?.profileTabsVue) {
-    initProfileTabs();
+    initUserProfileApp();
   } else {
     // eslint-disable-next-line no-new
     new UserTabs({ parentEl: '.user-profile', action });