Keymap: Alt-N to flip normals

Add shortcut since this has been removed from the context menu,
now it's in the mesh normals menu which isn't so convenient to access.

Shift-N is already used to recalculate normals,
this fits the convention of Alt removing/reversing.

release/scripts/presets/keyconfig/keymap_data/blender_default.py

@@ -3830,6 +3830,7 @@ def km_mesh(params):
          {"properties": [("inside", False)]}),
         ("mesh.normals_make_consistent", {"type": 'N', "value": 'PRESS', "shift": True, "ctrl": True},
          {"properties": [("inside", True)]}),
+        ("mesh.flip_normals", {"type": 'N', "value": 'PRESS', "alt": True}, None),
         ("view3d.edit_mesh_extrude_move_normal", {"type": 'E', "value": 'PRESS'}, None),
         op_menu("VIEW3D_MT_edit_mesh_extrude", {"type": 'E', "value": 'PRESS', "alt": True}),
         ("transform.edge_crease", {"type": 'E', "value": 'PRESS', "shift": True}, None),