Add support for CSS nesting

[wlipa]

Native browser support for css nesting is becoming quite widespread - https://caniuse.com/css-nesting

It appears that crass ignores nested rules. Would it be possible to support the nested syntax?

[rgrove]

Definitely possible, but I haven't yet looked into how extensive the changes would need to be to the parser. Thanks for letting me know you'd find this valuable!

[wlipa]

Thanks for the reply. I was thinking of using it to write a tool to nest / un-nest CSS.

[stoivo]

If we implement the next css spec, should it replace the current or do we expect to have both? I think it can replace since it backward compatible.

[rgrove]

The changes to the parser itself (and its public API) will be significant, so I would release this as a new major version (e.g. 2.0.0). Users who need to continue using the older parser for some reason can stick with 1.x.