fix: import cross-fetch conditionally

src/helper.ts

@@ -1,13 +1,11 @@
-import crossFetch from 'cross-fetch'
-
 type Fetch = typeof fetch
 
 export const resolveFetch = (customFetch?: Fetch): Fetch => {
   let _fetch: Fetch
   if (customFetch) {
     _fetch = customFetch
   } else if (typeof fetch === 'undefined') {
-    _fetch = crossFetch as unknown as Fetch
+    _fetch = async (...args) => await (await import('cross-fetch')).fetch(...args)
   } else {
     _fetch = fetch
   }