Add export for OverlaySynchronizer in olcs.ts

[luca-rath]

I want to pass a custom createSynchronizers function to the OLCesium constructor with the default OverlaySynchronizer, the default VectorSynchronizer and a custom RasterSynchronizer. I don't know why the OverlaySynchronizer isn't exported in the main file, if I import it from olcs/src/olcs/OverlaySynchronizer, there are typescript compilation errors, if I add the export to the olcs.ts file and import it from there, everything works just fine.

[gberaudo]

Thanks @luca-rath.