Enable NuGet publishing via GitHub Actions

Added a GitHub Actions workflow to automate publishing to NuGet.
The workflow triggers on `push` events for tags matching the
`*.*.*` pattern (semantic versioning) and supports manual
execution via `workflow_dispatch`. A `test` job was added
with a matrix strategy to ensure compatibility across multiple
operating systems.

Author: JunaMeinhold

.github/workflows/nuget.yml

@@ -4,6 +4,7 @@ on:
   push:
     tags:
       - '*.*.*'
+  workflow_dispatch:
 
 jobs:
   test: