Adding :scope pseudo-class support

[corneliusio]

This is actually not related to #901 perse, though it's possible it could inform it. I was thinking support for the :scope pseudo-class would make a lot of sense whether styles are scoped or global by default.

Currently, to apply styles directly to any root elements, some unique selector must be applied to it or them to do so. While this certainly isn't a real issue, it did make me start thinking if it should be necessary and how :scope might actually fit into Svelte from an ideological standpoint.

From what I can tell, pretty much everything is already present in svelte to more or less "flip a switch" for this kind of feature. I was able to play around a little and getting it working with just a few new lines of code.

But, I just wanted to throw this out there to see if anything stuck.

[antony]

Closing in favour of having a single place to discuss styling of child components