feat: add toBePressed matcher (#203)

[kretajak]

What:

New toBePressed matcher is added

Why:

Closes #203

Checklist:

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

In my opinion this PR could be followed by another one which would add support for checking aria-pressed="mixed". Maybe toBePartiallyPressed? It would follow the same approach as splitter toBeChecked and toBePartiallyChecked into two separate matchers.

[kretajak]

Hi, may I ask for reviews?

[kretajak]

Hi @gnapse, can I kindly ask for a review?

[gnapse]

Thanks for making this contribution. It is good stuff. Though there are a couple of things that need to be improved.

(the comments marked as "nit-pick" are not fundamental to be changed, but other ones are)

[gnapse]

@all-contributors please add @kretajak for code, test

[allcontributors]

@gnapse

I've put up a pull request to add @kretajak! 🎉

[github-actions]

🎉 This PR is included in version 6.7.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[kretajak]

I have created a follow-up with toBePartiallyPressed matcher.