Add GitHub action testing

.github/workflows/pull-request.yaml

@@ -0,0 +1,41 @@
+name: Demystifier PR Build
+on: pull_request
+
+jobs:
+  build:
+    name: "Build for PR"
+    runs-on: ${{ matrix.os }}
+    env:
+      DOTNET_NOLOGO: true
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [windows-latest, ubuntu-18.04, macOS-latest]
+        config: [Debug, Release]
+    steps:
+      - name: Clone source
+        uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+
+      - name: Install .NET Core SDK 2.1
+        uses: actions/setup-dotnet@v1
+        with:
+          dotnet-version: '2.1.x'
+
+      - name: Install .NET Core SDK 3.1
+        uses: actions/setup-dotnet@v1
+        with:
+          dotnet-version: '3.1.x'
+
+      - name: Install .NET SDK 5.0
+        uses: actions/setup-dotnet@v1
+        with:
+          dotnet-version: '5.0.x'
+
+      - name: Get .NET information
+        run: dotnet --info
+
+      - name: "Test"
+        run: dotnet test -c ${{ matrix.config }}
+