There was a topic asking how to do this, but it is locked due to inactivity and no one posted the correct solution. Here it is. I have only tested with bows and thrown, as I don’t have a gun or crossbow and am too cheap to buy crappy ones, but I’m sure it will work. It even uses the correct Shoot or Throw icon depending what you have equipped, though for some reason it takes about 30 seconds to update the icon after equipping the other weapon type.
EDIT: I realized there is a much more succinct method:
#showtooltip
/cast [equipped:thrown] Throw; Shoot
(Basically: If thrown equipped, throw. Otherwise, shoot.)
#showtooltip
/cast [equipped:bows] Shoot
/cast [equipped:guns] Shoot
/cast [equipped:crossbows] Shoot
/cast [equipped:thrown] Throw