Returns whether the character at this is within an editable region of text.
Non-editable text is “locked” and can’t be changed by the user via TextView. If no
tags applied to this text affect editability,
default_setting will be returned.
You don’t want to use this function to decide whether text can be inserted at this, because for insertion you don’t want to know whether the char at this is inside an editable range, you want to know whether a new character inserted at this would be inside an editable range. Use can_insert to handle this case.
true if text is editable by default
whether this is inside an editable range