chore: add GitHub <ci> action

This aims to replace our Travis CI builds.

.github/workflows/ci.yml

@@ -0,0 +1,28 @@
+name: ci
+on: [ push, pull_request ]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+    - name: Use Node.js
+      uses: actions/setup-node@v1
+      with:
+        node-version: '10.x'
+    - name: Cache Node.js modules
+      uses: actions/cache@v2
+      with:
+        # npm cache files are stored in `~/.npm` on Linux/macOS
+        path: ~/.npm
+        key: ${{ runner.OS }}-node-${{ hashFiles('**/package-lock.json') }}
+        restore-keys: |
+          ${{ runner.OS }}-node-
+          ${{ runner.OS }}-
+    - name: Install dependencies
+      run: npm ci
+    - name: Build
+      run: COVERAGE=1 npm run all
+    - name: Upload coverage
+      run: npx codecov