feat: Add Vitest integration for end users

[AliiiBenn]

Goal

Add native Vitest integration for end users to make type testing easier and more intuitive in Vitest projects.

Current Situation

Users currently have to use the library like this:

import { check, assert, expect } from '@deessejs/type-testing'

check<string>().equals<string>()
assert<{ a: string }>().hasProperty('a')

While this works, it's not integrated with their testing framework and feels disconnected from their test suites.

Proposed Solution

Create a /vitest module with custom Vitest matchers that integrate seamlessly with existing Vitest test suites.

Usage Example

import { expect, test } from 'vitest'
import { toHaveType, toBeType } from '@deessejs/type-testing/vitest'

expect.extend({ toHaveType, toBeType })

test('type checks', () => {
  expect<string>().toBeType<string>()
  expect<{ a: string }>().toHaveProperty('a')
  expect<string>().not.toBeType<number>()
})

Auto-setup Option

// vitest.config.ts
import { defineConfig } from 'vitest/config'
import { typeTestingMatchers } from '@deessejs/type-testing/vitest'

export default defineConfig({
  setupFiles: ['@deessejs/type-testing/vitest/setup'],
  test: {
    matchers: typeTestingMatchers()
  }
})

Implementation Plan

1. Create src/vitest/ directory
2. Implement custom matchers:
   • toBeType<T>() - Type equality check
   • toExtend<T>() - Type extends check
   • toHaveProperty(key) - Property existence check
   • toBeAny(), toBeNever(), toBeUnknown() - Special type checks
   • toBeNullable(), toBeOptional() - Nullability checks
   • toBeUnion(), toBeTuple(), toBeArray() - Structure checks
3. Add setup.ts for automatic matcher registration
4. Update documentation with Vitest examples
5. Add tests for the Vitest integration

Benefits

• Better DX: Seamless integration with existing Vitest test suites
• More intuitive: Familiar expect() syntax
• Type-safe: Full TypeScript support with autocomplete
• Framework-native: Feels like part of Vitest, not an external tool
• Adoption: Lower barrier to entry for Vitest users