CSS Outline Panel: show @layer as parent group

[lambdaschmied2]

When using layers in CSS they aren't used to group their child rules in the outline panel.

For example, when defining

@layer core, component, custom;

@layer core { ... }
@layer component { ... }
@layer custom { ... }

in my CSS file, the outline ignores those parent scopes and just shows its children.

Contrary to that, when using @media rules, they get represented in the outline.

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Adding feature this would make the outline easier to understand, when layers group the CSS rules by their topic.