Support video tag with poster attribute

[itmammoth]

Hi,
This PR adds support for the <video> tag by converting the poster attribute into a Markdown image.
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/video

Thanks.

Summary (Copilot generated )

This pull request adds support for converting HTML video elements to Markdown format in the markdownify library. The changes include a new conversion method and corresponding tests.

New feature: Video element conversion

• markdownify/__init__.py: Added a convert_video method to handle the conversion of HTML video elements to Markdown. This method checks for the presence of src and poster attributes and formats the output accordingly.

Testing the new feature

• tests/test_conversions.py: Added a test_video function to verify the correct conversion of video elements with different attributes to Markdown.

[AlexVonB]

Thanks for your contribution! If you have time for it, please check the change request.

One thing we should probably ignore is the <source> tag that can occur inside the video tag. But if you have a good idea how to include this as well, feel free to do so!