Computes the barycentric coordinates for a point with respect to a triangle.
Example:
// Returns Cartesian3.UNIT_X
const p = new Cesium.Cartesian3(-1.0, 0.0, 0.0);
const b = Cesium.barycentricCoordinates(p,
new Cesium.Cartesian3(-1.0, 0.0, 0.0),
new Cesium.Cartesian3( 1.0, 0.0, 0.0),
new Cesium.Cartesian3( 0.0, 1.0, 1.0));