Add exports field to package.json

See https://github.com/arethetypeswrong/arethetypeswrong.github.io/blob/main/docs/problems/FalseESM.md for more detail

package.json

@@ -6,8 +6,20 @@
   "repository": "git://github.com/ianstormtaylor/superstruct.git",
   "type": "module",
   "main": "./dist/index.cjs",
-  "module": "./dist/index.mjs",
+  "module": "./dist/index.js",
   "types": "./dist/index.d.ts",
+  "exports": {
+    ".": {
+      "import": {
+        "types": "./dist/index.d.ts",
+        "default": "./dist/index.js"
+      },
+      "require": {
+        "types": "./dist/index.d.cts",
+        "default": "./dist/index.cjs"
+      }
+    }
+  },
   "sideEffects": false,
   "files": [
     "dist"