Marks the nodes that can be placed directly into a
tableRow node.
In practice, this interface is redundant with
TableCellNode, as that abstracts
the common features of
tableHeader and
tableCell, which also happen to
be the only two node types that are usable within a
tableRow. However, they
serve different functional purposes in this library's type system, so they are both
included for consistency with the other interfaces that mark either common functionality
or the context within which they may be used.