GHA: add a Windows build

Author: janusw

.github/workflows/main.yml

@@ -9,10 +9,26 @@ on:
   workflow_dispatch:
 
 jobs:
-  build:
 
+  linuxBuild:
     runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Setup .NET Core
+      uses: actions/setup-dotnet@v4
+      with:
+        dotnet-version: |
+          3.1.x
+          5.0.x
+    - name: Install dependencies
+      run: dotnet restore src/GeoJSON.Net.sln
+    - name: Build
+      run: dotnet build src/GeoJSON.Net.sln --configuration Release --no-restore
+    - name: Test
+      run: dotnet test src/GeoJSON.Net.sln --no-restore --verbosity normal
 
+  winBuild:
+    runs-on: windows-latest
     steps:
     - uses: actions/checkout@v2
     - name: Setup .NET Core