Add support for native `@container` at-rule creation for container queries

[alexgrozav]

Summary

Add native support for generating CSS @container at-rules within the Styleframe TypeScript CSS API, enabling developers to use container queries for responsive design.

Motivation

Container queries and the @container at-rule allow components to adapt their styles based on the size of their parent containers rather than the viewport. This is increasingly important for building flexible, composable design systems.

Proposed Implementation

• Expose an API to define CSS container queries using the native @container at-rule.
• Ensure generated stylesheets include the correct container query syntax and integrate with other Styleframe features.
• Add documentation and usage examples for the new API.

References

• MDN: CSS @container

Acceptance Criteria

• Ability to create and compose container queries using Styleframe.
• Type-safe API for container query creation.
• Tests and documentation updated.