Add GitHub workflow

2 jobs have been specified:

- `pack` makes sure the package can be built.
- `lint` checks the code using Prettier.

.github/workflows/ci.yaml

@@ -0,0 +1,26 @@
+name: ci
+
+on:
+  pull_request:
+  push:
+    branches: [master]
+    tags: ['*']
+
+jobs:
+  pack:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-node@v2
+        with: { node-version: 14 }
+      - run: yarn --frozen-lockfile
+      - run: yarn pack
+
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-node@v2
+        with: { node-version: 14 }
+      - run: yarn --frozen-lockfile
+      - run: yarn lint