MAINT: Bring add_bookmark_dict and add_bookmark_destination together

See #1098

PyPDF2/_writer.py

@@ -1057,17 +1057,17 @@ def getNamedDestRoot(self) -> ArrayObject: # pragma: no cover
  def add_bookmark_destination(
  self, dest: PageObject, parent: Optional[TreeObject] = None
  ) -> IndirectObject:
- dest_ref = self._add_object(dest)
+ bookmark_ref = self._add_object(dest)
 
  outline_ref = self.get_outline_root()
 
  if parent is None:
  parent = outline_ref
 
  parent = cast(TreeObject, parent.get_object())
- parent.add_child(dest_ref, self)
+ parent.add_child(bookmark_ref, self)
 
- return dest_ref
+ return bookmark_ref
 
  def addBookmarkDestination(
  self, dest: PageObject, parent: Optional[TreeObject] = None
@@ -1103,8 +1103,7 @@ def add_bookmark_dict(
  if parent is None:
  parent = outline_ref
 
- parent = parent.get_object() # type: ignore
- assert parent is not None, "hint for mypy"
+ parent = cast(TreeObject, parent.get_object())
  parent.add_child(bookmark_ref, self)
 
  return bookmark_ref