Implement OnyxMenuItem

[MajaZarkova]

Why?

A follow up ticket on #833

Acceptance criteria

• [x] keyboard navigation is implemeneted

Definition of Done

• The following component requirements are implemented:
• [x] covered by component tests
• [x] screenshot tests are updated
• [x] follow-up tickets were created if necessary
• [x] Storybook can show the feature
• [x] Storybook code snippet of new/changed examples are checked that they are generated correctly
• [x] Set task to In Approval

Approval

• Storybook -> menu item is the inner component in the user menu
• [x] approved by designer @jannick-ux
• [x] approved by dev @larsrickert

Implementation details

<OnyxMenuItem>
    <OnyxListItem
        :selected="props.active"
        v-bind="menuButton?.listItem"
        class="onyx-future-nav-item"
        @click="emit('click')"
      >
        <a
          v-if="props.href"
          class="onyx-future-nav-item__anchor"
          v-bind="menuButton?.menuItem({ active: props.active })"
          :href="props.href ?? 'javascript:void(0)'"
        >
          <slot></slot>
        </a>
       <button
          v-else
          v-bind="menuButton?.menuItem({ active: props.active })"
       >
       </button
  </OnyxListItem>
</OnyxMenuItem>