Add CSS Nesting

[jonathantneal]

This adds css-nesting as a draft, taken from https://github.com/tabatkins/specs as css-nesting/index.bs and moved here as css-nesting/Overview.bs.

From #2701:

The Working Group just discussed css nesting, and agreed to the following:
RESOLVED: add css-nesting as an ED, Tab Atkins as editor, file issues until ppl are overall kinda happy with what it's like before we consider FPWD

This is original work by @tabatkins and should be reviewed by @tabatkins before being considered to merge.

Changes from the tabatkins/specs copy

• I renamed the bikeshed document from index.bs to Overview.bs to match other specs in this repository.
• I updated the top-most metadata section to match other specs, as well. This attempt may have included mistakes. @tabatkins, I would love the opportunity to learn of any mistakes that I could correct in future contributions.

Compliance with CONTRIBUTING.md

+@tabatkins
-@jonathantneal

[gsnedders]

@tabatkins is there any reason why we haven't merged this yet?

[svgeesus]

Metadata looks good to me.
Tab wrote it, so I see no need for his additional review.
WG approved moving this to ED, so approving.

[ademidun]

What's the status of this, please?

The draft proposed here looks good but it seems like I can't use it in my CSS files.

Sass and Less already support this. Might be a good idea to add it to CSS as well.

[SebastianZ]

The specification is in an early status. Discussions happen at https://github.com/w3c/csswg-drafts/labels/css-nesting.

There are currently no browsers that already support this already, not even experimentally, as far as I know.

Sebastian