[rhythmic-sizing] Implement layout logic for block-step-size

• Component: Layout and Rendering
• Status: NEW ---

[Sammy Gill]

https://drafts.csswg.org/css-rhythm/#block-step-size

This property defines the step unit for a block-level box’s block size. When the step unit is set to a positive <length>, the box’s outer height is rounded (see block-step-round) to the closest multiple of the unit. Negative <length> values are invalid.

Values other than none cause the box to establish an independent formatting context.

In situations where margins collapse, only the box’s own margin is considered in calculating its outer size.

When a box fragments, step sizing is applied per fragment.

[Radar WebKit Bug Importer]

<rdar://problem/105519481>