fix: make twMerge default to true in cn function

[jrgarciadev]

Closes: #268

Description

This PR makes twMerge default to true in the cn function, allowing users to call cn('bg-red-500', 'text-white') without explicitly passing { twMerge: true }. Users can still disable merging by passing { twMerge: false }.

Changes:

• Updated cn function in src/tw-merge.js to default twMerge to true when config is undefined or twMerge is not specified
• Added comprehensive tests in src/__tests__/cn.test.ts to verify default behavior and backward compatibility
• Added JSDoc documentation for cx, cn, tv, and createTV functions in src/index.d.ts
• Fixed type definition in src/lite.d.ts to match implementation

Before:

cn('bg-red-500', 'bg-blue-500')({ twMerge: true }) // Required

After:

cn('bg-red-500', 'bg-blue-500')() // twMerge defaults to true
cn('bg-red-500', 'bg-blue-500')({ twMerge: false }) // Can still disable### Additional context

• All existing tests pass, ensuring backward compatibility
• TypeScript type checking passes
• The change is minimal and follows the existing code patterns
• Documentation has been added to improve developer experience

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
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