Add GitHub action for linting (#8)

.github/workflows/lint.yml

+name: Lint
+
+on: pull_request
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v1
+
+      - name: Setup Node.js 12.x
+        uses: actions/setup-node@v1
+        with:
+          node-version: 12.x
+
+      - name: Install and lint
+        run: |
+          yarn
+          yarn lint

package.json

     "deploy": "gcloud app deploy --project=excalidraw-room",
     "fix": "yarn prettier --write",
     "gcp-build": "tsc -p .",
+    "lint": "yarn prettier --list-different",
     "prepare": "yarn gcp-build",
     "pretest": "yarn gcp-build",
     "prettier": "prettier \"**/*.{ts,md,json,yaml,yml}\"",
     "start": "node ./index.js",
-    "test": "yarn prettier --list-different"
+    "test": "yarn lint"
   },
   "version": "1.0.0"
 }