CI: Test type definitions

Also, fix a typo in the typings.

.github/workflows/test-type-definitions.yml

@@ -0,0 +1,21 @@
+name: Types
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+jobs:
+  lint:
+    name: Test type definitions
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - uses: actions/setup-node@v1
+      with:
+        node-version: '15'
+    - run: npm ci
+    - run: npm run test-type-definitions