navbar-vertical-next: prepare for new features

[spike-rabbit]

As shown in recent internal meetings, we will introduce some new features to the navbar-vertical. While the features are still in clarification, we already should start with some preparation:

• duplicate the navbar-vertical to a navbar-vertical-next (no code changes yes, just copy and necessary renaming)
• change menu items to declarative API / remove all the object input
• extract search into new component
• support for footer items

API Draft

<si-navbar-vertical>
  <si-navbar-search>
    <si-search-bar />
  <si-navbar-search>

  <si-navbar-items>
     <a si-navbar-item routerLink>...</a>
     <button si-navbar-item [siNavbarGroupTriggerFor]="groupTemplate">...</button>
  </si-navbar-items>

  <si-navbar-footer-items>
     <a si-navbar-item routerLink>...</a>
  </si-navbar-footer-items>
</si-navbar-vertical>