What problem does this feature solve?

Allow the filename and chunkFilename to use functions to align with the APIs from mini-css-extract-plugin. Currently, these can only be configured with string. Functions allow for more flexibility to handle special cases for generated files.

The rationale is the same as output.filename and output.chunkFilename, mentioned here:
#3483

What does the proposed API of configuration look like?

The new APIs would align with those used in mini-css-extract-plugin