Optional ReadonlystyleStyling for this run alone, overriding the node's.
Optional, because a caller writes these by hand and most runs in a rich
text are unstyled — requiring style: undefined on each of them is a word
per run that says nothing. SceneNode carries every key for the
opposite reason: it is built by this module and read by the encoder, and a
fixed shape is what lets that be a field read rather than a lookup.
ReadonlytextThe run's text.
One run of a rich-text node, with a style of its own.