Add workflow to create a NuGet package.

A fully-featured Release workflow would be too much for this repo.
Also enable package validation.

.github/workflows/pack.yml

@@ -0,0 +1,22 @@
+name: Pack
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  pack:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Pack
+        run: dotnet pack -c Release -o ./artifacts
+      - name: Upload
+        uses: actions/upload-artifact@v4
+        with:
+          name: artifacts
+          path: ./artifacts

Serilog.Sinks.MSBuild/Serilog.Sinks.MSBuild.csproj

@@ -23,6 +23,8 @@ When using this package, MSBuild's assemblies are not copied, which is not a pro
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <LangVersion>latest</LangVersion>
     <Nullable>enable</Nullable>
+    <EnablePackageValidation>true</EnablePackageValidation>
+    <PackageValidationBaselineVersion>1.5.0</PackageValidationBaselineVersion>
   </PropertyGroup>
 
   <ItemGroup>