…ncover

For instance, if an object to be hidden spans two background colors, you can specify `(fill: "000d", stroke: (bottom: rgb("fffd) + 0.2in))` to have the cover work even in that case

"width" and "height" can also be directly overridden in cases where their defaults don't work properly, e.g. with heavy strokes of slanted lines

This will more closely match the current default of `gray.lighten(50%)`

Convenient for one-time background colors that shouldn't incur a cumbersome registration to global state

This allows inline content to remain inline after being covered

…t size

Consider the case of `cover-with-rect(block(...))`. This should explicitly be in its own block, but adding a box deterministically inside `cover` will promote this to inline content. Providing an optional argument allows block-level content to stay that way if desired. "inline" is still a sensible default, since paragraph breaks around a box will allow it to be its own block anyway.