WIP: support tangents

[w0rm]

Closes #2

This adds support for tangents generation functionality:

normalMappedFaces :
    Obj.Decode.Decoder
        (
        TriangularMesh.TriangularMesh
            { position : Point3d.Point3d Length.Meters coordinates
            , normal : Vector3d.Vector3d Quantity.Unitless coordinates
            , uv : ( Basics.Float, Basics.Float )
            , tangent : Vector3d.Vector3d Quantity.Unitless coordinates
            , tangentBasisIsRightHanded : Basics.Bool
            }
        )

It also introduces an example that uses unpublished elm-3d-scene https://unsoundscapes.com/nefertiti/