fix: fix the .d.cts default export type

[kricsleo]

resolves #457

[kricsleo]

I've tested against the ./example/1.zero-config and the output of utils.d.cts has changed from:

declare function sum(a: number, b: number): number;

export { sum as default };

to:

declare function sum(a: number, b: number): number;

export = sum;

This is the expected types in pair with the utils.cjs output:

'use strict';

function sum(a, b) {
  return a + b;
}

module.exports = sum;