Add GitHub Actions workflow for testing and publishing

Added a `nuget.yml` workflow to automate testing and publishing
of a .NET project to NuGet. The workflow triggers on `push`
events for tags matching semantic versioning patterns (`*.*.*`).

The workflow includes two jobs:
- `test`: Runs on a matrix of OS environments (Ubuntu, Windows,
  macOS) to restore dependencies, build the project, and run
  tests using multiple .NET versions (6.0.x, 7.0.x, 8.0.x, 9.0.x).
- `publish`: Runs after successful tests, builds the project,
  creates a NuGet package, and pushes it to NuGet.org using
  a secure API key from GitHub Secrets.

Author: JunaMeinhold

.github/workflows/nuget.yml

@@ -0,0 +1,62 @@
+name: Publish to NuGet
+
+on:
+  push:
+    tags:
+      - '*.*.*'
+
+jobs:
+  test:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+    
+    steps:
+    - uses: actions/checkout@v4
+    
+    - name: Setup .NET
+      uses: actions/setup-dotnet@v4
+      with:
+        dotnet-version: |
+          6.0.x
+          7.0.x
+          8.0.x
+          9.0.x
+    
+    - name: Restore dependencies
+      run: dotnet restore
+    
+    - name: Build
+      run: dotnet build --no-restore --configuration Release
+    
+    - name: Run tests
+      run: dotnet test --no-build --configuration Release --verbosity normal
+
+  publish:
+    needs: test
+    runs-on: ubuntu-latest
+    
+    steps:
+    - uses: actions/checkout@v4
+    
+    - name: Setup .NET
+      uses: actions/setup-dotnet@v4
+      with:
+        dotnet-version: |
+          6.0.x
+          7.0.x
+          8.0.x
+          9.0.x
+    
+    - name: Restore dependencies
+      run: dotnet restore
+    
+    - name: Build
+      run: dotnet build --no-restore --configuration Release
+    
+    - name: Pack Hexa.NET.Utilities
+      run: dotnet pack Hexa.NET.Utilities/Hexa.NET.Utilities.csproj --no-build --configuration Release --output ./artifacts
+    
+    - name: Push to NuGet
+      run: dotnet nuget push ./artifacts/*.nupkg --api-key ${{ secrets.NUGET_API_KEY }} --source https://api.nuget.org/v3/index.json --skip-duplicate