Add GH Actions CI workflow

.github/workflows/ci.yml

@@ -0,0 +1,40 @@
+name: CI
+
+on:
+  pull_request:
+  push: 
+    branches: 
+      - master
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - uses: actions/setup-node@v1
+        with:
+          node-version: '12'
+
+      # https://github.com/actions/cache/blob/master/examples.md#node---yarn
+      - name: Get yarn cache directory path
+        id: yarn-cache-dir-path
+        run: echo "::set-output name=dir::$(yarn cache dir)"
+
+      - name: Cache yarn
+        uses: actions/cache@v2
+        id: yarn-cache
+        with:
+          path: ${{ steps.yarn-cache-dir-path.outputs.dir }}
+          key: ${{ runner.os }}-yarn-${{ hashFiles('**/yarn.lock') }}
+          restore-keys: |
+            ${{ runner.os }}-yarn-
+
+      - name: Install dependencies
+        run: yarn install
+
+      - name: Check all required files are present
+        run: bin/check_required_files_present
+
+      - name: Run JS tests
+        run: yarn test