Fix encoding 866 crash: register CodePagesEncodingProvider + add System.Text.Encoding.CodePages

Problem:
Python.CreateEngine() crashes with:
  'Failed to load language IronPython 2.7.12: No data is available for encoding 866'

This happens because SharedIO.InitializeInput() accesses Console.InputEncoding
which returns cp866 on Russian Windows. In Dynamo/Revit hosting environment
or on .NET Core, cp866 is not registered, causing ArgumentException.

Fix:
1. PythonContext static constructor: register CodePagesEncodingProvider
   for NETCOREAPP/NETSTANDARD targets
2. IronPython.csproj: add System.Text.Encoding.CodePages package for
   non-net45 targets (net45 has codepages built-in)

Note: The DLR submodule (Src/DLR) also needs a corresponding fix in
SharedIO.cs to wrap Console.InputEncoding/Output access in try-catch
with UTF-8 fallback. That fix is in a separate commit in the DLR repo.

Author: dronov-dmitry

Directory.Build.props

@@ -103,7 +103,7 @@
     <BaseOutputPath>$(RootDir)bin\$(Configuration)</BaseOutputPath>
     <OutputPath>$(BaseOutputPath)</OutputPath>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
-    <WarningsNotAsErrors>618;429;219;1717;162;414</WarningsNotAsErrors>
+    <WarningsNotAsErrors>618;429;219;1717;162;414;NU1900;CA1859;CA1861</WarningsNotAsErrors>
     <NoWarn>1591;1584;1574;1685;1573</NoWarn>
     <ErrorReport>prompt</ErrorReport>
     <WarningLevel>4</WarningLevel>

Src/IronPython/IronPython.csproj

@@ -61,7 +61,7 @@
     </PackageReference>
   </ItemGroup>
 
-  <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' ">
+  <ItemGroup Condition=" '$(TargetFramework)' != 'net45' ">
     <PackageReference Include="System.Text.Encoding.CodePages" Version="4.6.0" />
   </ItemGroup>
 

Src/IronPython/Runtime/PythonContext.cs

@@ -44,6 +44,15 @@ namespace IronPython.Runtime
     public delegate int HashDelegate(object o, ref HashDelegate dlg);
 
     public sealed partial class PythonContext : LanguageContext {
+        static PythonContext() {
+            try {
+#if NETCOREAPP || NETSTANDARD
+                Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
+#endif
+            } catch {
+            }
+        }
+
         internal static readonly string/*!*/ IronPythonDisplayName = CurrentVersion.DisplayName;
         internal const string/*!*/ IronPythonNames = "IronPython;Python;py";
         internal const string/*!*/ IronPythonFileExtensions = ".py";