feat: add story layout prop

[davestewart]

Overview

This PR adds layout props to Story components:

<Story :layout="{ type: 'grid', cols: 3 }">...</Story>

It attempts to mirror our used functionality from Histoire, but also:

• correctly-calculates widths and gaps for arbitrary column amounts in CSS
• supports a new cols value (rather than entering percentages)
• supports the old <width>px value (fixes pixel value and wraps)
• supports the old <width>% value (calculates cols and wraps)
• the type: grid value is not strictly required; can just enter { cols: 4 } and it is inferred

It also makes some decisions on styling:

• variant header styling is no-wrap + truncated
• default padding gap is now 1.5rem

Screenshots

[timhanlon]

Amazing, thanks!

[davestewart]

One thing it could do with would be a default layout.

Perhaps we can do that with runtime config.