fix: Add support for TS 4.7+ node16/next module

in TS 4.7+, TS introduced a node16/next module mode.

in this mode, if libraries are using "exports" field on package.json,
each "exports" field needs "types" field, or fail to load typings.

for more information, see release note: https://devblogs.microsoft.com/typescript/announcing-typescript-4-7/#esm-nodejs

Author: rinsuki

package.json

@@ -8,8 +8,12 @@
   "unpkg": "./dist/react-intersection-observer.umd.js",
   "types": "./dist/index.d.ts",
   "exports": {
-    "./test-utils": "./dist/test-utils.js",
+    "./test-utils": {
+      "types": "./dist/test-utils.d.ts",
+      "default": "./dist/test-utils.js"
+    },
     ".": {
+      "types": "./dist/index.d.ts",
       "require": "./dist/react-intersection-observer.js",
       "default": "./dist/react-intersection-observer.modern.mjs"
     }