<svelte:body/> Allow adding classes to the body tag

[Bradley-H]

Describe the problem

This should be a rather straight forward answer, as far as i'm aware the only way to add classes, id's to the body tag is to do an onMount(() =>{
const body = document.querySelector("body").
// apply logic here
})

And if you are using dynamic classes you're going to have to add a bunch of if statements that is tedious

Describe the proposed solution

My solution I think quite simple

instead of the above, we do this

<Svelte:body class="{whatever}"/>

Alternatives considered

N/A

Importance

would make my life easier

[PatrickG]

Duplicate #3105

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