get(file = null) {
        if (file) {
            return path.posix.join(
                this.mix.config.publicPath,
                this.manifest[this.normalizePath(file)]
            );
        }

        return Object.fromEntries(
            Object.entries(this.manifest).sort((a, b) => a[0].localeCompare(b[0]))
        );
    }
Get the underlying manifest collection.
@param {string|null} [file]
@returns {string | Record<string, string>}