fix: export defaultConfig as value and remove responsiveVariants

[jrgarciadev]

Closes: #273

Description

This PR fixes the TypeScript error reported in #273 where defaultConfig cannot be used as a value because it was exported using export type. Additionally, it removes the responsiveVariants feature which is no longer supported in Tailwind CSS v4.

Changes:

1. Fixed defaultConfig TypeScript export issue

   • Added explicit value export declaration in index.d.ts to allow defaultConfig to be imported and used as a value
   • Removed defaultConfig from types.d.ts and config.d.ts to avoid conflicts with export type * re-exports
   • Fixes the error: 'defaultConfig' cannot be used as a value because it was exported using 'export type'

2. Removed responsiveVariants feature

   • Removed responsiveVariants property from config.js (no longer supported in Tailwind CSS v4)
   • Removed getScreenVariantValues function and all responsive variants logic from core.js (~105 lines removed)
   • Simplified getVariantValue to handle only string variant keys
   • Object variant keys (previously used for responsive variants) are now ignored

3. Added comprehensive tests

   • Created defaultConfig.test.ts with 10 test cases covering:
     • Value import/export verification
     • Property modifications (twMerge, twMergeConfig)
     • Integration with tv() and createTV() functions
     • Nested config modifications

Additional context

• This PR addresses GitHub issue TS error while defining global config #273
• All 164 tests pass
• TypeScript compilation succeeds without errors
• The removal of responsiveVariants aligns with Tailwind CSS v4's removal of config.content.transform support

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Follow the Style Guide.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).