The action taken for each key, named as KeyboardEvent.key reports it; a key whose browser default
is to survive is left out of the map, which a caller assembles as the state of the moment requires
A keyboard handler taking the key it is given an action for, and claiming it from the browser default
Maps keys to what they do.
Declares keyboard behaviour as the actions a component offers rather than as the dispatch selecting among them, and settles with the browser which keys the component has taken over.