delegate library Null safety

TODO: Remove this file in the next breaking release, because implementation files should be located in the src folder, https://bit.ly/3fA23Yz.

Classes

EditorTextSelectionGestureDetectorBuilder
EditorTextSelectionGestureDetectorBuilderDelegate

Typedefs

CustomStyleBuilder = TextStyle Function(Attribute attribute)
EmbedBuilder = Widget Function(BuildContext context, Embed node, bool readOnly)