FLocalPlanarSimplify::IsFlat

The below helper functions are used by SimplifyAlongEdges, but also the similar customized versions of the same algorithm in [FMeshBoolean](API\Runtime\GeometryCore\Operations\FMeshBoolean) and [FMeshSelfUnion](API\Runtime\GeometryCore\Operations\FMeshSelfUnion) as well Test if the triangles connected to a vertex are all coplanar