ci: add lint action

Author: pd4d10

.github/workflows/lint.yml

@@ -0,0 +1,10 @@
+on:
+  push:
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    container:
+      image: google/dart:latest
+    steps:
+      - uses: actions/checkout@v1
+      - run: cd highlight && dartanalyzer .