Add async vtk.wasm examples with web worker and vtkAlgorithm abort

• Adds two new examples demonstrating usage of web workers in vtk.wasm. They both use vtkAlgorithm abort functionality to enable more responsive VTK web applications.
• The basic example AsyncClipper uses a main() function and the -sPROXY_TO_PTHREAD link flag which does all the heavy lifting of running main() itself on a new thread and transferring the canvas for offscreen rendering.
• The second example WrappedAsyncClipper doesn't have a main() and is entirely driven from javascript through custom bindings. It shows in detail how to transfer the canvas offscreen to the newly created pthread.

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