Add example of generation via webworker

[gkjohnson]

Add an example demonstrating how to create a BVH via webworker -- maybe even provide a utility out of the box.

import { asyncGenerateMeshBVH } from 'three-mesh-bvh';

asyncGenerateMeshBVH( geometry, { copyBuffers: false } )
    .then( ( { geometry, bvh } ) => {

        // geometry is available to be used again
        geometry.boundsTree = bvh;

    } );

[Ely-S]

:+1: