Replaces Acorn over Travis CI by ESLint over (Microsoft) GitHub Actions

> See #11, Acorn was great but not sufficient for Mozilla's libPref syntax...

.github/workflows/linting.yml

@@ -0,0 +1,17 @@
+---
+name: Linting
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    name: Run ESLint on user.js
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - uses: actions/setup-node@v1
+
+      - run: npm install -g eslint
+      - run: eslint user.js