Creates and returns a CharacterRun at the beginning of this Element. The text to be inserted is specified as well as whether this should be a new character run or part of the existing one.
String representing the text to insert.
boolean representing if this should have default formatting and become its own character run (true) or if it should assume the formatting of the first character in this Element, becoming a part of that first character run.
CharacterRun containing the inserted text.