Adds an (empty) sculpt mode for Grease Pencil v3 objects. The object `SCULPT_GPENCIL` mode is re-used for Grease Pencil object types. A `SCULPT_GREASE_PENCIL` context mode has been added, which is specific to grease pencil objects. This is necessary for polling tools and keymaps. Pull Request: https://projects.blender.org/blender/blender/pulls/119338