Configure .snupkg symbol package generation

We want to continue publishing SourceLink-ed debug symbols, but using
classic Windows PDBs generates a huge package. Using the much smaller
portable PDB format would be much better, but some people may still
experience problems with those. (See e.g. GitHub issue devlooped#428.)

Let's follow the official guidance (see links below) and publish debug
symbols in a separate `.snupkg` package to NuGet's symbol server. This
means that we can (must, in fact) use portable PDBs as it will be the
consumers' choice whether to use the separate symbols or not.

References:
 * https://docs.microsoft.com/en-us/nuget/create-packages/symbol-packages-snupkg
 * https://github.com/dotnet/SourceLink#alternative-pdb-distribution

src/Moq/Moq.csproj

@@ -9,7 +9,7 @@
  <TargetFrameworks>net45;netstandard2.0</TargetFrameworks>
  <AssemblyName>Moq</AssemblyName>
  <DebugSymbols>True</DebugSymbols>
- <DebugType>full</DebugType>
+ <DebugType>portable</DebugType>
  <DocumentationFile>bin\$(Configuration)\$(TargetFramework)\$(AssemblyName).xml</DocumentationFile> 
  <GenerateDocumentation>true</GenerateDocumentation>
  <NoWarn>0419</NoWarn>
@@ -33,6 +33,8 @@
  <PackageOutputPath>$(OutputDirectory)</PackageOutputPath>
  <PackageTags>moq;tdd;mocking;mocks;unittesting;agile;unittest</PackageTags>
  <RepositoryUrl>https://github.com/moq/moq4</RepositoryUrl>
+ <IncludeSymbols>True</IncludeSymbols>
+ <SymbolPackageFormat>snupkg</SymbolPackageFormat>
  </PropertyGroup>
 
  <PropertyGroup Condition=" '$(TargetFramework)' == 'net45' ">