Add ES5 validation

This makes sure that the main library code is ES5 compatible and ES6
features aren't used accidentally.

.github/workflows/build.yml

@@ -68,6 +68,12 @@ jobs:
       run: node js/meshopt_encoder.test.js
     - name: test simplifier
       run: node js/meshopt_simplifier.test.js
+    - name: check es5
+      run: |
+        npm install -g es-check
+        npx es-check es5 js/meshopt_decoder.js js/meshopt_encoder.js js/meshopt_simplifier.js
+        npx es-check --module es5 js/meshopt_decoder.module.js js/meshopt_encoder.module.js js/meshopt_simplifier.module.js
+        npx es-check es5 gltf/library.js
 
   gltfpack:
     runs-on: ubuntu-latest