feat: @import CSS at-rule supports both layer and supports

[ngtr6788]

Implements #8670.

Right now, layer is supported on all major browsers, but supports is supported by Firefox only. Also, I decided to implement this CSS rule on the css-tree fork on this repo so that we can have this now, until css-tree has a better way to support this rule. 🙃

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🦋 Changeset detected

Latest commit: 15bef57

The changes in this PR will be included in the next version bump.

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[ngtr6788]

From csstree, csstree/csstree#259 (comment), the maintainer @lahmatiy says that this feature should be coming soon on the next major version. In the meantime, I will close this PR and wait until the new version is released.