Fullscreen support
An equivalent to Twine 1's fullscreen()
function, which queries the browser and asks for fullscreen mode depending on the provided value, as well as supplying the current fullscreen status.
The main question is whether or not this should be (set-fullscreen:) and (get-fullscreen:) command macros, two on/off setters and a getter, or a special "environment variable". I've been pushing for getter-only environment values like (passage:) to be macros, but I don't have a consistent model for getter and setter environment values.