LineSegements2: Add a "threshold" Raycaster param

[gkjohnson]

Adds a threshold raycaster param like Lines uses so line segments 2 can be expanded by a pixel width for raycasting.

const raycaster = new Raycaster();
raycaster.params.Line2 = { threshold: 2 };

// raycast...

ping @WestLangley

[mrdoob]

Thanks!