`mapjs/src/browser/dom-map-widget.js`: Using deprecated keyCode within document listener

[s6mike]

https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/code

https://beta.workflowy.com/#/b3ab0251f2e5

[s6mike]

mapjs/src/browser/dom-map-widget.js: 190

Also see: #4

[s6mike]

Should use KeyboardEvent.key instead.