Subgrid

A value for `grid-template-columns` or `-rows` that makes a nested grid adopt its parent's tracks instead of defining new ones. It solves the alignment problem that used to force layout data to be passed down through component props, or fixed heights to be hard-coded. Support is now broad, but the mental model catches people out: the subgrid participates in the parent's track sizing, so its content can change the parent's columns.