Improve TypeScript support for $$restProps, class/style as functions

[rgossiaux]

Because these components spread $$restProps all over the place, the TypeScript support for props besides those exported by the component is lacking. Concretely that means:

• Props which <Render> uses which come via $$restProps: class and style--particularly when using them as functions.
• Props which are spread all the way down to the underlying HTML elements.

It would be nice if we could do this semi-automatically (sveltejs/language-tools#1363), but we can also use $$Props to type the component manually. Ideally we can get support for some similar $$RestProps type of interface (suggesting this to the language-tools folks) but if not we can still use $$Props--it'll just be a bit more painful since we now have to worry about forgetting to add any props there.