Add support for native `@layer` at-rule creation

[alexgrozav]

Summary

Add built-in support for generating native CSS @layer at-rules within the Styleframe TypeScript CSS API. This will enable users to structure their CSS with layers for improved specificity management and future compatibility.

Motivation

CSS @layer is increasingly used for managing cascade and specificity. Supporting this natively will allow developers to leverage modern CSS features and organize their Design Systems code more effectively in Styleframe.

Proposed Implementation

• Expose an API to define CSS layers using the native @layer at-rule.
• Ensure generated stylesheets include the correct layer syntax and can be composed with other Styleframe features.
• Add documentation and usage examples for the new API.

References

• MDN: CSS @layer

Acceptance Criteria

• Ability to create and compose CSS layers using Styleframe.
• Type-safe API for layer creation.
• Tests and documentation updated.