Add test documents

[alejandrgaspar]

Hello, first of all thank you for this project. I have been using the library you wrote and it has been very useful. I am interested in contributing to solve some needs that have arisen and to keep typst versions up to date.

However, before making any contributions, I want to ensure that I'm not breaking anything, so I was thinking of adding the sample documents as tests, but noticed that there was no defined way to declare development dependencies, to add for example pytest.

Personally, I find that using Poetry to manage these dependencies is the most comfortable way. Nonetheless, I'd greatly appreciate your input on this matter.

Thank you for your time, and I look forward to your insights.