refactor: split cn into cn and cnMerge functions

[jrgarciadev]

Description

This PR splits the cn function into two separate functions (cn and cnMerge) to eliminate the Proxy complexity that was causing errors. The previous implementation used a Proxy to support dual usage patterns (direct usage and config-based usage), which introduced complexity and type issues.

Changes:

• cn: Simplified function that returns a string directly with default twMerge config. No function call needed - just cn("px-2", "px-4") returns "px-4".
• cnMerge: New function that supports custom twMerge config via a second function call. Usage: cnMerge("px-2", "px-4")({twMerge: false}) returns "px-2 px-4".

Implementation details:

• Extracted shared merge logic into executeMerge helper function
• Removed Proxy implementation entirely
• Updated core.js to use cnMerge for config-based calls (passed as parameter to getTailwindVariants)
• Updated all tests to reflect the new API
• Added comprehensive documentation in README explaining the difference between cx, cn, and cnMerge

Additional context

This is a breaking change for users who were calling cn(...)() with an empty call or relying on the Proxy's string coercion. However, the common use case of cn("...") now works more intuitively by returning a string directly. Users who need custom config should migrate to cnMerge.

The split provides:

• Better TypeScript support (no Proxy types)
• Clearer API intent
• Better performance (no Proxy overhead)
• Easier maintenance

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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