feat: support the `@counter-style` directive

[sno2]

Implements the @counter-style directive per the spec.

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[devoncarew]

Thanks for the contribution! Adding @leonsenft for thoughts.

@sno2 - please also bump the pubspec version to match (and the version there and in the chaneglog should likely be 0.17.4-wip).

[mosuem]

Closing as the dart-lang/csslib repository is merged into the dart-lang/tools monorepo. Please re-open this PR there!