feat: add custom twMergeFn support to createTV

[chioio]

Description

Summary

• Add support for custom twMergeFn in createTV function
• Allow users to pass pre-configured twMerge instances
• Runtime config takes precedence over createTV config

Changes

• Added twMergeFn option to TWMConfig type
• Updated cn function to use custom merge function when provided
• Enhanced createTV to properly handle custom merge functions
• Added comprehensive tests for all scenarios

Test plan

• All existing tests pass
• New tests cover custom twMergeFn functionality
• Type checking passes
• Linting passes

Additional context

---

What is the purpose of this pull request?

Support custom tailwind merge function for createTV

• 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).

[chioio]

This is my scenario:

import { twMerge } from '@weapp-tailwindcss/merge'
import { createTV } from 'tailwind-variants'

export const tv = createTV({
  twMergeFn: twMerge,
})

weapp need escape symbols.