Add some layout primitives

[planktonic]

Utility classes can be used to create some complex layouts, but it’d be great to have some commonly-used layouts encapsulated in CSS classes. That would avoid having to recreate them in every codebase and template/component system.

Some possibilities listed below. Probably these would be best implemented using some global spacing and line-height variables — those can be used to ensure everything is spaced consistently.

• vertically stack and separate content. Apply a consistent vertical gap or margin to all the direct children
• row/column switch. Put content in a row (e.g. a row of action buttons), that switches to a column layout when there’s not enough available width