chore: use vue-demi for backward compat with vue2

[messenjer]

Use vue-demi - https://github.com/MaxLeiter/sortablejs-vue3/issues/69

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Name	Status	Preview	Comments	Updated
sortablejs-vue3	✅ Ready (Inspect)	Visit Preview	💬 Add your feedback	Feb 10, 2023 at 9:44AM (UTC)

[messenjer]

Can you adjust the README to mention vue 2 compatibility?

done, also

[MaxLeiter]

Thanks!