Consider adding "ManifoldEvaluator"

[gkjohnson]

As an alternative to the existing "Evaluator" function a "ManifoldEvaluator" function could be added that internally implements CSG operations using manifold-3d. Manifold would bring a guarantee of manifold mesh operations while retaining the convenience of the current CSG API.

class ManifoldEvaluator {

  evaluate( brushA, brushB, operations, targetBrushes ) {

    // - convert brush a and b to a manifold instances
    // - execute manifold csg operations
    // - convert manifold instances to three.js brushes

  }

}

Further operations could include keeping around the manifold instances to improve performance when running subsequent operations.