FR: New sexp that toggles lightshafts (similar to the toggle motion debris sexp)

[wookieejedi]

Request from shiv and Matt to have a new sexp that allows FREDers to toggle lightshafts on/off for missions. There is the existing toggle motion debris sexp which does this for motion debris, so it could follow a similar naming convention.

[xShivan]

Thank you for raising this for us 👍