@layer support

[argyleink]

bit of a trickier one, as this PR currently only covers half of the functionality.

https://developer.mozilla.org/en-US/docs/Web/CSS/@layer

todo:

• add layer() function import syntax
• support anonymous layers
• support nested layers
• add tests?

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[andreamah]

Hi! Would you mind adding some tests to css-spec.coffee for this? Since it's a new area.

[andreamah]

Oops, I just noticed that you listed tests in your todo. Could you write some really basic ones before merging this? Just to have it there.

[eliabexp]

add @scope support please