[HTML5] Enforce JavaScript style with eslint.

Applies to javascript files inside the platform library folder, the
exposed Engine code, and any javascript files in modules.

Files ending with ".externs.js" will be ignored, you can create a
".eslintignore" file to specify extra files to be ignored.

.github/workflows/static_checks.yml

@@ -9,7 +9,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v2
 
-      # Azure repositories are not reliable, we need to prevent azure giving us packages.
+      # Azure repositories are not reliable, we need to prevent Azure giving us packages.
       - name: Make apt sources.list use the default Ubuntu repositories
         run: |
           sudo rm -f /etc/apt/sources.list.d/*
@@ -33,6 +33,12 @@ jobs:
         run: |
           bash ./misc/scripts/black_format.sh
 
+      - name: JavaScript style checks via ESLint
+        run: |
+          cd platform/javascript
+          npm ci
+          npm run lint
+
       - name: Documentation checks
         run: |
           doc/tools/makerst.py --dry-run doc/classes modules