Add NerdBank.GitVersioning support

This commit adds [NerdBank.GitVersioning](https://www.nuget.org/packages/Nerdbank.GitVersioning), which gives every build of plist-cil a unique version number.

The base version number is stored in `version.json` (e.g. 2.2) and is incremented by 1 every time a commit is added.  Builds off branches contain a `+g{commit}` suffix, indicating that these builds are prerelease.

This makes it easier to keep track of different versions of plist-cil.

.github/workflows/ci.yaml

@@ -23,10 +23,13 @@ jobs:
     - name: Create NuGet packages
       run: |
         dotnet pack -c Release -o nuget/
+    - name: Get version information
+      uses: dotnet/nbgv@master
+      id: nbgv
     - name: Publish NuGet packages as artifacts
       uses: actions/upload-artifact@v3
       with:
-        name: plist-cil
+        name: plist-cil-${{ steps.nbgv.outputs.SemVer2 }}
         path: nuget/
     - name: Publish NuGet packages to NuGet feed
       run: |