This commit adds the `BKE_brush_duplicate` function that performs a deep
copy of a brush ID and all associated IDs into the current Main
database.
Unit tests are added with this case to ensure that grandchildren and
embedded data is handled correctly for both Sculpt and Grease Pencil
brushes, as the latter contains more ID references than other brush
types.
Related to #138105
Pull Request: https://projects.blender.org/blender/blender/pulls/138629
44e36ef581