Add GitHub Actions workflow for NuGet package build

.github/workflows/pull_request_main.yml

@@ -0,0 +1,22 @@
+name: Build NuGet package on main branch pull request
+
+on:
+  pull_request:
+    branches:
+      - "main"
+    paths:
+      - "TimetableDesigner.Backend.Events**"
+
+jobs:
+  build:
+    name: Build
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6.0.2
+      - name: Setup .NET
+        uses: actions/setup-dotnet@v3
+        with:
+          dotnet-version: 10.0.x
+      - name: Build
+        run: dotnet build