Ability to bind attributes & classes to <svelte:body>

[GavHern]

Describe the problem

I recently tried to use a class directive on the <svelte:body> special element. It didn't work, and I read up on the docs to realize it's only for binding events. I feel like it would be a nice feature to have.

Describe the proposed solution

Allow binding classes/attributes to the svelte:body special element.

Alternatives considered

You could achieve this using javascript but then you're leaving the svelte ecosystem and dealing with the issues a front-end framework is there to solve.

Importance

nice to have

[baseballyama]

Duplicate of #3105

And already there is PR.
#6928