Use `rollupOptions.options.assetFileNames` when specified in the Vite config

Typically in vite we use build.rollupOptions.output.assetFileNames to tweak output directory and file names, like:

            rollupOptions: {
                output: {
                    assetFileNames: (asset) => {
                        if (
                            ['.jpg', '.png', '.svg', '.avif', '.webp'].some((ext) =>
                                asset.name?.endsWith(ext),
                            )
                        ) {
                            return 'images/[name]-[hash][extname]';
                        }
                        return 'assets/[name]-[hash][extname]';  // for other assets like fonts and audios etc.
                    },
                },

While this plugin provides an assetsDir option, it should still conform to the idiom above when assetsDir is not specified. But unfortunately that’s not the current behavior.

I can submit a PR for this if feasible.