Add wrapped wasm example using web worker and vtkAlgorithm abort

- This example adds on the AsyncClipper example by demonstrating how one
 might use embind and web workers.
- The main difference is that with embind, there is no `main(int, char**)`
 function and the entire application is driven from javascript.
- This example shows in detail how to create a pthread and transfer the
 canvas offscreen so that the new web worker can render into it. In the
 `AsyncClipper`, we used `-sPROXY_TO_PTHREAD` link flag which did all the
 tasks necessary to run our `main(int, char**)` on a new web worker.