Add ESM support for browser environments

Author: EzraBrooks

package.json

@@ -6,7 +6,11 @@
     ".": {
       "types": "./index.d.ts",
       "import": "./index.mjs",
-      "require": "./index.js"
+      "require": "./index.js",
+      "browser": {
+        "import": "./dist/eventemitter3.esm.js",
+        "require": "./dist/eventemitter3.umd.js"
+      }
     },
     "./package.json": "./package.json"
   },