Make a separate publicPath for chunks and assets in splitting mode

## Problem
When I build an application to run it on nodejs I get errors when using `publicPath: "/"` because all chunks get the same `publicPath` in `import`. Using `publicPath: "."` will not work, because in this case all assets get this `publicPath` and become inaccessible on pages other than the main page.

## Solution
Separate `publicPath` for chunks and for assets. To maintain backward compatibility, it is possible to make `chunkPublicPath` optional (and use `publicPath` in its absence). What do you think?

Start by tracing how publicPath is applied in splitting mode, especially for imports, chunks, and assets when running on nodejs. Done would mean evaluating the proposed optional chunkPublicPath behavior and its fallback to publicPath while preserving backward compatibility.

Written by the indexing model from the issue text.