Create a new CSS @layer for octicons

[joshblack]

When styles downstream in Primer React interact with an icon, and .octicon is present on the page, the .octicon call will take precedence since it is in the anonymous layer and Primer React styles are brought in through the primer-react layer.

This change updates the .octicon class to be wrapped in a @layer named primer.icons. We should make sure this layer appears first to avoid any overrides in the future.

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Latest commit: ccd66d9

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Pull Request Overview

This PR introduces a new CSS layer to encapsulate octicon styles and ensure they take precedence over downstream Primer React styles.

• Wraps the .octicon class in a @layer primer.icons block.
• Guarantees the primer.icons layer is first to avoid unintended overrides.