ci: Migrate bundle size bot from Travis to GitHub actions

.github/workflows/pr-size.yml

@@ -0,0 +1,45 @@
+name: "PR size bot"
+on: [pull_request]
+
+jobs:
+  build:
+    name: PR size bot
+    runs-on: ubuntu-latest
+    steps:
+      - name: Setup node
+        uses: actions/setup-node@v1
+        with:
+          node-version: 14.x
+
+      - name: Checkout PR branch
+        uses: actions/checkout@v1
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Record PR size
+        run: |
+          mkdir .size
+          npx lerna bootstrap --ignore @bugsnag/expo --ignore @bugsnag/react-native
+          npx lerna run build --scope @bugsnag/browser
+          cat packages/browser/dist/bugsnag.min.js | wc -c > .size/after-minified
+          cat packages/browser/dist/bugsnag.min.js | gzip | wc -c > .size/after-gzipped
+
+      - name: Checkout base branch
+        uses: actions/checkout@v1
+        with:
+          ref: ${{ github.base_ref }}
+          clean: false
+
+      - name: Record target branch size
+        run: |
+          npm ci
+          npx lerna bootstrap --ignore @bugsnag/expo --ignore @bugsnag/react-native
+          npx lerna run build --scope @bugsnag/browser
+          cat packages/browser/dist/bugsnag.min.js | wc -c > .size/before-minified
+          cat packages/browser/dist/bugsnag.min.js | gzip | wc -c > .size/before-gzipped
+
+      - name: Run danger
+        uses: danger/danger-js@9.1.6
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}