Add the `locationHelper._RectMixin.union` method. (#20705)
Summary of the issue:
Sometimes we may need to create a rectangle that contains all other rectangles, such as a paragraph rectangle created from multiple text lines within the same paragraph.
Description of user facing changes:
Description of developer facing changes:
Add a method to create a rectangle that contains all other rectangles.
Description of development approach:
Add the locationHelper._RectMixin.union method.
