Add `<List>` component

[haydn]

Objective

Add <List> and <ListItem> components:

const List = ({ children, format }) => (
  <Stack
    indent
    style={{
      listStyleType: format === "ordered" ? "decimal" : format === "unordered" ? "bullet" : null,
    }}
    tagName={format === "ordered" ? "ol" : "ul"}
  >
    {children}
  </Stack>
);

const ListItem = ({ children }) => <li style={{ listStyleType: "inherit" }}>{children}</li>;

Rationale

This makes it cleaner/easier to put together simple lists:

<List ordered>
  <ListItem>This is an item.</ListItem>
</List>