Curves: initial brush system integration for curves sculpt mode

This adds the boilerplate code that is necessary to use the tool/brush/paint
systems in the new sculpt curves mode.

Two temporary dummy tools are part of this patch. They do nothing and
only serve to test the boilerplate. When the first actual tool is added,
those dummy tools will be removed.

Differential Revision: https://developer.blender.org/D14117

release/scripts/modules/bl_keymap_utils/keymap_from_toolbar.py

@@ -189,6 +189,7 @@ def generate(context, space_type, *, use_fallback_keys=True, use_reset=True):
                             'VERTEX_GPENCIL': "gpencil_vertex_tool",
                             'SCULPT_GPENCIL': "gpencil_sculpt_tool",
                             'WEIGHT_GPENCIL': "gpencil_weight_tool",
+                            'SCULPT_CURVES': "curves_sculpt_tool",
                         }.get(mode, None)
                     else:
                         attr = None