Create a GitHub action to run yarn build on all PRs and pushes to main

Fixes #251

---

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.github/workflows/yarn-build.yml

@@ -0,0 +1,28 @@
+name: Yarn Build
+
+on:
+  pull_request:
+    branches:
+      - main
+  push:
+    branches:
+      - main
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v2
+        with:
+          node-version: '14'
+
+      - name: Install dependencies
+        run: yarn install
+
+      - name: Run yarn build
+        run: yarn build