Dev SSR loader should track CSS dependencies

[Rich-Harris]

At present, the dev server inlines 'critical' CSS which basically means this:

const rendered = options.root.render(props);

// ...
const styles = `<style>${rendered.css.code}</style>`

Leaving aside the lack of sourcemaps here, this doesn't totally work, because it only includes Svelte's own CSS. It doesn't include anything that was manually imported.

The dev loader just ignores CSS files altogether, but it could instead gather up those dependencies and report back

[Rich-Harris]

Turns out external CSS files also fail in build, during the optimisation step. Currently blocked by this so working on it

[Rich-Harris]

fixed by #279 and #286