Updates to Alert component on Web

[janogarcia]

• [ ] Changed: Set description text to font.body.sm.regular
• [ ] Added: New variant with a stacked layout. Actions on the bottom.
• [ ] Added: New variant with a 2 column layout. Actions on the right column, vertically centered. It should shift to stacked layout when the component width (container query) is below 600px.
• [ ] Bug: The corner radius should be 8px.
• [ ] Bug: The close/dismiss button is not correctly sized.

Figma — Alert Component

[germain-gg]

For https://github.com/vector-im/compound/issues/250