chore: add es version check

bin/checkES5.js

@@ -0,0 +1,17 @@
+// ensure output files are ES5
+
+const fs = require('fs');
+const acorn = require('acorn');
+
+const files = ['./client/index.js', './server/index.js', './server/loadMeta.js'];
+
+console.log('Ensuring files are ES5:', files);
+files.forEach(file => {
+    const contents = fs.readFileSync(file, 'utf-8');
+    try {
+        acorn.parse(contents, { ecmaVersion: 5 });
+    } catch (e) {
+        console.error(`Error: ${file} is not ES5:`, e);
+        process.exit(1);
+    }
+})

package.json

@@ -49,9 +49,10 @@
     "update": "./bin/update.sh",
     "upgrade": "./bin/upgrade.sh",
     "build": "./bin/build.sh",
-    "test": "mocha test/ -R dot"
+    "test": "mocha test/ -R dot && node ./bin/checkES5.js"
   },
   "devDependencies": {
+    "acorn": "^8.11.3",
     "chai": "^4.3.4",
     "google-closure-compiler": "^20230802.0.0",
     "google-closure-library": "^20230802.0.0",