Output modern code with ESM

package.json

@@ -1,6 +1,6 @@
 {
   "name": "fflate",
-  "version": "0.3.0",
+  "version": "0.3.1",
   "description": "High performance (de)compression in an 8kB package",
   "main": "lib/index.js",
   "module": "esm/index.js",

tsconfig.esm.json

@@ -3,7 +3,7 @@
   "compilerOptions": {
     "declaration": false,
     "moduleResolution": "node",
-    "module": "ESNext",
+    "target": "ES2018",
     "outDir": "esm"
   }
 }