Add RoslynVersion configuration in project

[atifaziz]

Objective

Implement multi-Roslyn configuration support in src/DocoptNet/DocoptNet.csproj so the project can build analyzer outputs for baseline Roslyn (3.10.0) and Roslyn 4.4.0, with safe packing behavior.

Scope

• Add RoslynVersion-conditioned property group for Roslyn 4.4 build isolation.
• Keep baseline behavior unchanged when RoslynVersion is not set.
• Split Microsoft.CodeAnalysis.CSharp package reference into conditional versions (3.10.0 vs 4.4.0).
• Add pack item for analyzers/dotnet/roslyn4.4/cs/DocoptNet.dll.
• Add pre-pack validation target that errors if Roslyn 4.4 analyzer output is missing.

Files

• src/DocoptNet/DocoptNet.csproj

Tasks

• Add conditioned PropertyGroup for RoslynVersion == 4.4:
  • Set BaseOutputPath to an isolated Roslyn 4.4 path.
  • Set BaseIntermediateOutputPath to an isolated Roslyn 4.4 path.
  • Add DefineConstants entry for ROSLYN4_4.
• Replace unconditional Microsoft.CodeAnalysis.CSharp reference with two conditioned references:
  • 3.10.0 for default/empty RoslynVersion.
  • 4.4.0 when RoslynVersion is 4.4.
• Add second analyzer pack item for analyzers/dotnet/roslyn4.4/cs using MSBuild property-based paths.
• Add _ValidateRoslyn44AnalyzerOutput target (BeforeTargets="GenerateNuspec") with clear remediation message.

Acceptance Criteria

• dotnet build at repo root still produces baseline artifacts with no RoslynVersion required.
• dotnet build src/DocoptNet/DocoptNet.csproj -f netstandard2.0 -p:RoslynVersion=4.4 succeeds and emits isolated output.
• dotnet pack src/DocoptNet/DocoptNet.csproj fails with explicit guard message if Roslyn 4.4 analyzer output is absent.
• Changes do not alter lib/ target framework outputs.

Dependencies

• None.

Notes

• Use project-relative properties ($(MSBuildProjectDirectory), $(BaseOutputPath), etc.) and avoid hard-coded path separators.

[atifaziz]

Closed via PR #213.