Use classes instead of attributes for scoping

[Rich-Harris]

Classes are always faster than attributes. We should consider using them:

<!-- this... -->
<div class='svelte-xyz123'>...</div>
<div class='svelte-xyz123 foo'>...</div>

<!-- ...instead of this: -->
<div svelte-xyz123>...</div>
<div svelte-xyz123 class='foo'>...</div>

It will complicate <div class='{{whatever}}'> slightly, because we'd now need to include the scoping string in the className that it gets updated with.

[constgen]

Attributes are main reason why I don't use scoped styles in Svelte. Classes really should be used for this.

[PaulBGD]

If we did something like #890 and deprecated passing a string (maybe allowing passing an array?) we could in a non-backwards compatibility way have no issue with making this change.

[Rich-Harris]

This is implemented in 1.57