fix: update duplicate checking script regex and deduplication logic

[KRUSHAL2956]

Pull Request Description

Closes #1291. Upgrades the regular expression and logic inside scripts/check-duplicates.mjs to correctly match:

1. Indented class definitions (e.g. inside media queries).
2. Classes with pseudo-classes (e.g. :hover, :active, :focus-visible).
3. Grouped class selectors (e.g. .class1, .class2).

To prevent false duplicate detections from multiple states/rules in the same file (such as .ease-btn, .ease-btn:hover, and .ease-btn:active all residing inside components/buttons.css), the script now deduplicates class name definitions at the file level. A class name is only flagged as a duplicate if it is defined across multiple different files in the core and components directories.

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Type of Change

• ✨ New animation / hover effect
• 🧩 New component
• 📝 Documentation improvement
• 🐛 Bug fix in an existing submission (duplicate checker script and its baseline config)
• Other

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Submission Checklist

⚠️ PRs that fail this checklist will be closed without review.
Note: This checklist is geared towards new community example submissions. Since this is a tooling/maintenance bug fix resolving an open issue (#1291), changes to scripts/ are required.

• All changes are inside submissions/examples/your-feature-name/
• Includes demo.html — self-contained, opens in browser with no server
• Includes style.css — raw CSS for the proposed feature
• Includes README.md — what it does, how to use it, why it fits EaseMotion CSS
• 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?

It improves the duplicate detector script to recognize indented selectors, pseudo-classes, and grouped selectors, while ignoring multiple declarations of the same class in a single file.

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Notes for Maintainer

All styles have been validated using npm run lint and all unit tests pass.
The baseline configuration was updated to reflect the cross-file duplicate ease-btn-hover which is defined in both core/animations.css and components/buttons.css (the latter overrides the transition when inside preferred reduced motion settings).