First pass on boundary visuals

[lachlansleight]

To encourage players to walk around the playspace boundaries before the begin interacting with the particles at the beginning of intro mode.

For now this can be simple and particle-based: create a prefab with two particle systems, one box emitter (x = 0.05, y = 0.05, z = 0.2) where particles barely move (so it creates a line of particles), and one box emitter where they shoot up and downwards in a starting burst, color over lifetime = fade. As the player walks near prefab, turn of emission on first system, and trigger second system's burst. Once second system is done, disable prefab GO.