Use Vector3.UP as a default value for look_at's up vector

[aaronfranke]

Vector3.UP is the most common argument to use for look_at, so it would be a nice idea to have it as the default argument.

Some statistics from the Godot demo projects, 10 out of 13 use Vector3.UP, or 4 out of 5 if we exclude the IK demo:

$ grep -RIni "look_at("
3d/ik/addons/sade/ik_fabrik.gd:154:                             bone_nodes[i].look_at(get_bone_transform(i+1).origin + skeleton.global_transform.origin, Vector3.UP)
3d/navmesh/navmesh.gd:55:                       robot.look_at(look_at_point, Vector3.UP)
viewport/gui_in_3d/gui_3d.gd:167:               node_area.look_at(look, Vector3.UP)

$ grep -RIni "looking_at("
3d/platformer/enemy/enemy.gd:64:                get_node("Armature").set_transform(Transform().looking_at(-dir, up))
3d/ik/addons/sade/ik_fabrik.gd:286:                             bone_trans = bone_trans.looking_at(b_target.origin + dir, Vector3.UP)
3d/ik/addons/sade/ik_fabrik.gd:296:                             bone_trans = bone_trans.looking_at(b_target.origin, Vector3.UP)
3d/ik/addons/sade/ik_fabrik.gd:322:                     bone_trans = bone_trans.looking_at(b_target.origin + dir, Vector3.UP)
3d/ik/addons/sade/ik_look_at.gd:87:             rest = rest.looking_at(target_pos, Vector3.RIGHT)
3d/ik/addons/sade/ik_look_at.gd:89:             rest = rest.looking_at(target_pos, Vector3.UP)
3d/ik/addons/sade/ik_look_at.gd:91:             rest = rest.looking_at(target_pos, Vector3.FORWARD)
3d/ik/addons/sade/ik_look_at.gd:93:             rest = rest.looking_at(target_pos, Vector3.UP)
3d/waypoints/waypoint.gd:70:            var look = camera_transform.looking_at(parent_translation, Vector3.UP)
misc/matrix_transform/marker/AxisMarker3D.gd:15:        holder.transform = holder.transform.looking_at(translation, Vector3.UP)

[akien-mga]

Thanks!