Consolidate icons

[Rich-Harris]

At the moment there are two different approaches to icons in use:

• importing .svg files is great because you can use them in CSS rules, and it's very easy to do things like rotating chevrons. but you lose the ability to use currentColor (meaning stuff has to be duplicated for dark/light mode), and we have icons duplicated between apps/svelte.dev and the two packages
• <Icons> lets us use currentColor, but I don't love the mechanism — every page load embeds every icon, and a bunch of them probably aren't used. We can't just keep adding junk to it over time, it's not sustainable. And it increases DOM size — we need an <svg> and a <use> for every icon, instead of just a background image

For the icons themselves, some are from the Figma document, but since it doesn't have everything we need I've started bringing stuff in from Lucide. We could consider using lucide-svelte instead of our own Icons.svelte, but I haven't looked into what the overhead is.

Anyway it'd be nice to settle on a single consistent approach if we can.