Add es-check to verify builds produce valid es5 code

.escheckrc

@@ -0,0 +1,10 @@
+{
+    "ecmaVersion": "es5",
+    "modules": "false",
+    "files": "./dist/**/*.js",
+    "not": [
+        "./dist/libraries/pdf.worker.js",
+        "./dist/libraries/worker-bundle.js",
+        "./dist/libraries/wasm-gen/libarchive.js"
+    ]
+}