insert 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

AutoExitBlockRule
Heuristic rule to exit current block when user inserts two consecutive newlines. [...]
AutoFormatLinksRule
CatchAllInsertRule
InsertEmbedsRule
InsertRule
PreserveBlockStyleOnInsertRule
Preserves block style when user inserts text containing newlines. [...]
PreserveInlineStylesRule
PreserveLineStyleOnSplitRule
ResetLineFormatOnNewLineRule