Mapping Start Event
class MappingStartEvent @JvmOverloads constructor(val anchor: Anchor?, val tag: String?, val implicit: Boolean, val flowStyle: FlowStyle, val startMark: Mark? = null, val endMark: Mark? = null) : CollectionStartEvent
Marks the beginning of a mapping node.
This event is followed by a number of key value pairs.
The pairs are not in any particular order. However, the value always directly follows the corresponding key.
After the key value pairs follows a MappingEndEvent.
There must be an even number of node events between the start and end event.