bugfix for `.keyboard-raycastable' added unsafely

[kylebakerio]

fixes bug described here and shown here. You can remix that glitch and swap out commented line 193/192 to see the effect of this change there.

Basically:

"".split(",")
// -> [""]
[""].push(".selector")
// -> ["",".selector"]
["",".selector"].join(",")
// -> ",.selector"

raycaster then interprets ",.selector" as a faulty css selector down the line in raycaster.js line 152, here:

      // note: data.objects === ",.selector"
      
      this.el.sceneEl.querySelectorAll(data.objects)