Attempt to resolve a GitHub CI Actions isue: CI is currently failing because restore can't find NUnit and friends. The error message and google search indicate that GitHub Actions is only looking in the "Microsoft Visual Studio Offline Packages" cache. Add a nuget.config file to in an attempt to tell GitHub Actions to query nuget.org in addition to the offline cache.

.gitignore

@@ -22,6 +22,7 @@ src/generator/generator
 *.opensdf
 *.pdb
 *.config
+!nuget.config
 *.vcxproj
 *.filters
 *.sln

nuget.config

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <packageSources>
+    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
+  </packageSources>
+</configuration>