fix: proper vitest ts support (#515)

[nyaapass]

What:

This PR extends Vitest's default Assertion interface to fix the toBeInTheDocument ts error when using Vitest (#515).

Why:

I don't know (still cannot reproduce #515 on my own computer). But from this comment, adding

/// <reference types="@testing-library/jest-dom" />

in custom vitest-setup.ts file would fix this error in that situation. Therefore I think the types/vitest.d.ts may need to be updated.

And when I install this repo's dependencies by pnpm and running tsc -p types/__tests__/vitest, there would be similar errors of #515 (if using npm or import '../../jest' instead of import '../../vitest' like that comment, there would be no error). This PR would fix this error even using pnpm.

How:

https://vitest.dev/guide/extending-matchers.html

Checklist:

• Documentation
• Tests
• Updated Type Definitions
• Ready to be merged

[ecoms-ye]

Confirmed in my office computer (a pnpm monorepo with the With Vitest and With TypeScript setup):

• Switch to the fork version by changing package.json to "@testing-library/jest-dom": "https://github.com/nyaapass/jest-dom", the error can be fixed:
  • 
• When back to the original @testing-library/jest-dom@6.4.2 version, the Upgrading to v6.0.0 remove the toBeInTheDocument interface with vitest #515 error would be shown again:
  •