Add test in .NET Framework 4.8.1

.github/workflows/test-netstandard.yml

@@ -4,11 +4,24 @@ on: pull_request
 
 jobs:
   test-netstandard:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
 
     strategy:
       matrix:
-        netstandard-version: ['netstandard2.0', 'netstandard2.1']
+        include:
+          - os: ubuntu-latest
+            netstandard-version: 'netstandard2.0'
+            test-tfm: 'net8.0;net10.0'
+          - os: ubuntu-latest
+            netstandard-version: 'netstandard2.1'
+            test-tfm: 'net8.0;net10.0'
+          - os: windows-latest
+            netstandard-version: 'netstandard2.0'
+            test-tfm: 'net481'
+
+    defaults:
+      run:
+        shell: bash
 
     steps:
     - uses: actions/checkout@v4
@@ -24,7 +37,7 @@ jobs:
       run: |
         cd src
         sed -i 's/<TargetFrameworks>.*<\/TargetFrameworks>/<TargetFrameworks>${{ matrix.netstandard-version }}<\/TargetFrameworks>/' Markdig/Markdig.targets
-        sed -i 's/<TargetFrameworks>.*<\/TargetFrameworks>/<TargetFrameworks>net8.0;net10.0<\/TargetFrameworks>/' Markdig.Tests/Markdig.Tests.csproj
+        sed -i 's/<TargetFrameworks>.*<\/TargetFrameworks>/<TargetFrameworks>${{ matrix.test-tfm }}<\/TargetFrameworks>/' Markdig.Tests/Markdig.Tests.csproj
         echo "Markdig.targets TFMs:"
         grep "TargetFrameworks" Markdig/Markdig.targets
         echo "Markdig.Tests.csproj TFMs:"