Add support for Scale transition

[ekhaled]

https://github.com/ekhaled/svelte-transitions-scale

Would you be open to including this as an official transition under the svelte-transitions banner?

I am willing to transfer the repository under the sveltejs user if you want. I can also provide a pull request to this repo's Readme file, if you allow the transfer. I'm also open to other suggestions.

Caveats:

The name svelte-transitions-scale is already taken on npm (which I didn't realise until I tried to publish it).

However, my version allows scaling to arbitrary factors, not just to scale(1). Because from and to are both configurable.

I also think from and to are more terse than initial.

Whatever you decide, I think we need a "blessed" scale transition plugin.