Fix handling of style scoping and class: with spread scopes

[Conduitry]

Fixed #3790 (which also fixes #2721 in a better way than I mentioned on that ticket).

In DOM mode for elements that need the scoping class and have spread attributes, this adds toggle_class(node, "svelte-xyz", true); calls to the hydrate and update blocks, rather than trying to add or update a class= attribute in the element's AST. This prevents the scoping class and the spread class from clobbering one another.

In SSR mode for elements that need the scoping class and/or have class: directives and have spread attributes, this combines the scoping class and/or the class: directives into a string of classes that need to be added to the element after all of the spreading. This string is passed as a new second argument to spread(), which uses it to update or set the class attribute it's been given. This prevents multiple class= attributes from being emitted by the code.