Test netstandard (#915)

* Add GH Action to test netstandard 2.0 and 2.1

* Account for TFM changes in tests project

.github/workflows/test-netstandard.yml

@@ -0,0 +1,44 @@
+name: Test netstandard
+
+on: pull_request
+
+jobs:
+  test-netstandard:
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        netstandard-version: ['netstandard2.0', 'netstandard2.1']
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Setup .NET
+      uses: actions/setup-dotnet@v4
+      with:
+        dotnet-version: |
+          8.0.x
+          9.0.x
+
+    - name: Patch build to test ${{ matrix.netstandard-version }}
+      run: |
+        cd src
+        sed -i 's/<TargetFrameworks>.*<\/TargetFrameworks>/<TargetFrameworks>${{ matrix.netstandard-version }}<\/TargetFrameworks>/' Markdig/Markdig.targets
+        sed -i 's/<TargetFrameworks>.*<\/TargetFrameworks>/<TargetFrameworks>net8.0;net9.0<\/TargetFrameworks>/' Markdig.Tests/Markdig.Tests.csproj
+        echo "Markdig.targets TFMs:"
+        grep "TargetFrameworks" Markdig/Markdig.targets
+        echo "Markdig.Tests.csproj TFMs:"
+        grep "TargetFrameworks" Markdig.Tests/Markdig.Tests.csproj
+
+    - name: Restore dependencies
+      run: dotnet restore src/Markdig.Tests/Markdig.Tests.csproj
+
+    - name: Test Debug
+      run: |
+        dotnet build src/Markdig.Tests/Markdig.Tests.csproj -c Debug --no-restore
+        dotnet test src/Markdig.Tests/Markdig.Tests.csproj -c Debug --no-build
+
+    - name: Test Release
+      run: |
+        dotnet build src/Markdig.Tests/Markdig.Tests.csproj -c Release --no-restore
+        dotnet test src/Markdig.Tests/Markdig.Tests.csproj -c Release --no-build

src/markdig.slnx

@@ -2,7 +2,6 @@
   <Folder Name="/Build/">
     <File Path="../.editorconfig" />
     <File Path="../.gitattributes" />
-    <File Path="../.github/workflows/ci.yml" />
     <File Path="../.gitignore" />
     <File Path="../changelog.md" />
     <File Path="../license.txt" />
@@ -10,6 +9,10 @@
     <File Path="Directory.Packages.props" />
     <File Path="global.json" />
   </Folder>
+  <Folder Name="/Build/GitHub Actions/">
+    <File Path="../.github/workflows/ci.yml" />
+    <File Path="../.github/workflows/test-netstandard.yml" />
+  </Folder>
   <Project Path="Markdig.Benchmarks/Markdig.Benchmarks.csproj">
     <BuildDependency Project="Markdig/Markdig.csproj" />
   </Project>