refactor 2d plane support

[mreinstein]

there is currently 1 function, ray-plane-distance, but I have other 2d plane functions sitting around in various places:

• signedDistanceTo(plane, point)
• isFrontFacingTo(plane, dir)
• several functions for constructing a 2d plane from different inputs (segment, another plane)

It would be nice to consolidate these, and expose a 2d plane primitive. Currently leaning torwards:

{
        origin (vec2),
        normal (vec2),
        D: (float),
}