Add decoders code to dist

[ivmartel]

Decoders code is provided by adding the decoders folder to the package without pre-processing. Webpack allows to process and include this code as part of the dist when creating workers when they are created with new Worker(new URL('./worker.js', import.meta.url));.
This would also mean that passing the scripts path when clients initialise dwv becomes unnecessary. The synchronous buffer decoding (SynchPixelBufferDecoder class) also becomes obsolete.