feat(dialogs/spawnDialog): return dialog instance and unmount function

[ShGKme]

☑️ Resolves

Missing feature from the stable8.

• Add return to dialogs/spawnDialog with 2 values:
  • vm - mounted dialog component public instance
    • Allows calling public methods
  • unmount - function to programmatically unmount the dialog

It misses typing. Will be added in the next PR.

Alternative

If we say, vm and unmount should never be needed, we can return Promise resolved with the close result.

🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable
• 2️⃣ Backport bugfixes to stable8 for maintained Vue 2 version.