Modal improvements

[Atmos4]

Scrolling improvements

• overflow on modal instead of article
• prevent body overflow when modal is open

Animations

• animations on open and close. Uses allow-discrete to prevent the modal from being focusable when closed. Browser support is low but non-support will just result in graceful degradation (no animation on closing modal).

This PR is more of a POC. I can split it into 2 PRs, one for the scrolling and one for the animations.

I am also unsure which animations I should include. Maybe it is just safer to explain in the docs how to add animations easily rather than enforcing specific animations.