Fired when composition starts inside one of the editables.
Introduced by CompositionObserver.
CompositionObserver
Note that because CompositionObserver is attached by the EditingView this event is available by default.
EditingView
args : tuple
module:engine/view/observer/compositionobserver~ViewDocumentCompositionStartEvent#args
name : 'compositionstart'
module:engine/view/observer/compositionobserver~ViewDocumentCompositionStartEvent#name