IconBase: support all valid svg props

[AnxiousDarkly]

so there are two issues blocking this from working properly, the first is that spreading props directly via $$props does not work, resulting in a read-only error:

Cannot assign to read only property '[insert propery name]' of object '#<SVGImageElement>

the second is that svelte does not support passing scoped class selectors to children, see: sveltejs/svelte#2870