feat: add mixin support and application utility to tss

[pradeep0153]

Introduces mixin.ts to the tss engine, allowing developers to define reusable partial styles (Mixin) and apply them using the applyMixin utility. This sets the foundation for more DRY, SCSS-like styling configurations within TermUI applications. Resolves #1612.

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Reviewing files that changed from the base of the PR and between 4b874c0 and 1e1be33.

📒 Files selected for processing (1)

• packages/tss/src/mixin.ts

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🧪 Generate unit tests (beta)

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[github-actions]

Hi @pradeep0153 👋

⭐ Star this repo before your PR merges.

Why? GSSoC 2026 contributors who star get priority review and points credit. After you star, push any commit (or re-run this check). The needs-star label lifts automatically.

Thanks for your contribution to TermUI.