feat: add SCSS variables and mixins to reduce repetitive CSS

[S0412-2007]

Pull Request Description

Adds SCSS variables and mixins (_variables.scss, _mixins.scss) to centralize easing curves and duration tokens, eliminating repetitive cubic-bezier and transition declarations. Fixes #1116.

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Type of Change

• ✨ New animation / hover effect
• 🧩 New component
• 📝 Documentation improvement
• 🐛 Bug fix in an existing submission
• Other (DRY refactor / SCSS tooling enhancement)

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Submission Checklist

• All changes are inside scss/
• Includes demo.html
• Includes style.css
• Includes README.md
• No changes to core/
• No changes to components/
• One feature per PR (no bundled unrelated changes)

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Feature Description

What does this add?
_variables.scss with easing curve and duration tokens, and _mixins.scss with ease-transition() and ease-animation() helper mixins.

How does a developer use it?

@import 'scss/variables';
@import 'scss/mixins';

.my-element {
  @include ease-transition(transform);
}

Why does it fit EaseMotion CSS?
Eliminates repeated cubic-bezier and transition declarations across the codebase, reduces bundle size and keeps the code DRY.

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Demo

• Demo added (demo.html works by opening directly in a browser)

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Browser Testing

• Chrome
• Firefox
• Edge
• Safari (optional but appreciated)

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Notes for Maintainer

Non-breaking SCSS tooling addition. Existing compiled CSS is completely unchanged — this only adds SCSS source files for developers using a build pipeline.

Closes #1116

[S0412-2007]

Hi @SAPTARSHI-coder ,
I have fixed the conflicts and ckecks, kindly request you to review and merge the same.
Thank you.