Declare "module" field in package.json

This change exposes the ESM build of react-testing-library.

Author: maisano

.all-contributorsrc

@@ -615,6 +615,15 @@
       "contributions": [
         "code"
       ]
+    },
+    {
+      "login": "maisano",
+      "name": "Richard Maisano",
+      "avatar_url": "https://avatars2.githubusercontent.com/u/689081?v=4",
+      "profile": "https://github.com/maisano",
+      "contributions": [
+        "code"
+      ]
     }
   ]
 }