fix: opt for default export in package.json

NOTE:
some compiler may not recognize the `import`
key in package.json's exports field, so changing to
default for a better compatibility

package.json

@@ -38,8 +38,8 @@
   "types": "lib/index.d.ts",
   "exports": {
     ".": {
-      "import": "./lib/index.js",
-      "types": "./lib/index.d.ts"
+      "types": "./lib/index.d.ts",
+      "default": "./lib/index.js"
     },
     "./package.json": "./package.json"
   },