feat(slots): add additional slots in Tabs

[nknwns]

Hi!

This is a really useful feature that allows you to build tabs more flexibly. For example, if we need to specify what kind of tab group it is in one line before the tabs.

<tabs>
  <template #before-tabs>My requests</template>
  <tab name="Active">
    First tab content
  </tab>
  <tab name="Passive">
    Second tab content
  </tab>
</tabs>