Add missing file IAstRuleReturnScope`1.cs into sources for CSharp2 Antlr3.Runtime

Interface source file IAstRuleReturnScope`1.cs was missing from source directory for CSharp2 directory and, thus, causing the "Antlr3.Runtime (VS2008).csproj" project to fail to build. This fix creates this file (copied from the equivalent location in the CSharp3 runtime source directory). Interface definition should work just fine with CSharp2 runtime.
Generate project files for VS2010 for CSharp2 runtime

Duplicates VS2008 projects and converts to VS2010 format by importing into Visual Studio 2010