bugfix: exempt the :root psuedo-selector from the random css prefix

[davemo]

• the change in :not(...) styles are broken #1705 introduced a small bug for users who were relying on assigning global CSS variables via the :root selector
• this change adds a small exemption to avoid prefixing the :root pseudo-selector with the random prefix that svelte adds

here is a screenshot that illustrates the bug (pre 3.6.3 [working], post 3.6.3 [broken]), and the corresponding source in my App.svelte component file:

<style>
  :root {
    --unchecked-box: '\02610';
    --checked-box: '\02611';
    --snowman: '\02603';
  }
</style>

[Rich-Harris]

Thanks — the purist in me chafes at treating :root differently to other pseudo-classes, but the pragmatist can't see any reason you wouldn't want this behaviour