feat: add definePagicOptions methods for improve ux

[kyaruary]

use a definePagicOptions methods instead of export define plain object that can improve ux

origin pagic.config.ts

export default {
 // no prompt
  srcDir: "docs/"
}

now

import { definePagicOptions } from 'https://deno.land/x/pagic/mod.ts';
export default definePagicOptions({
  // got prompt!
 srcDir: "docs/"
});

[xcatliu]

Import PagicConfig would be the same.