Add API Analyzer Assembly (#906)

* Start on API analyzers

* Finish GuildAccessAnalyzer

* Update build script (will this do?)

* Correct slashes

* Extrapolate DerivesFromModuleBase() to an extension method

* Quick refactoring

* Add doc file

test/Discord.Net.Tests/AnalyzerTests/Extensions/AppDomainPolyfill.cs

@@ -0,0 +1,30 @@
+using System.Linq;
+using System.Reflection;
+using Microsoft.DotNet.PlatformAbstractions;
+using Microsoft.Extensions.DependencyModel;
+
+namespace System
+{
+    /// <summary> Polyfill of the AppDomain class from full framework. </summary>
+    internal class AppDomain
+    {
+        public static AppDomain CurrentDomain { get; private set; }
+
+        private AppDomain()
+        {
+        }
+
+        static AppDomain()
+        {
+            CurrentDomain = new AppDomain();
+        }
+
+        public Assembly[] GetAssemblies()
+        {
+            var rid = RuntimeEnvironment.GetRuntimeIdentifier();
+            var ass = DependencyContext.Default.GetRuntimeAssemblyNames(rid);
+
+            return ass.Select(xan => Assembly.Load(xan)).ToArray();
+        }
+    }
+}