Need some layout helpers: Box and Stack components

[jstansbe]

We need a stack component, and utility helpers, to create a mui like <Stack /> component. This makes it easy to flow content as row/column and set item's horizontal and vertical alignment.
mui stack component https://mui.com/material-ui/react-stack/

To enable this, we also need a <Box /> component (which would probably enable us to make a bunch of things including the stack as described above).
mui stack component https://mui.com/material-ui/react-box/