add fern check github action (#218)

Co-authored-by: Patrick Thornton <patrick@Ferns-MacBook-Pro.local>

Author: patrickthornton

.github/workflows/fern_check.yml

@@ -0,0 +1,20 @@
+name: Fern Check
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  run:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Install Fern
+        run: npm install -g fern-api
+
+      - name: Check API is valid
+        run: fern check