fix: move BaseIntermediateOutputPath to Directory.Build.props to fix MSB3539 warning

The MSB3539 warning occurs because BaseIntermediateOutputPath and
BaseOutputPath were being set in DocoptNet.csproj, which is too late
in the MSBuild evaluation order. These properties must be set before
Microsoft.Common.props is imported. Directory.Build.props is the
correct location since it's imported early enough.

Co-authored-by: atifaziz <20511+atifaziz@users.noreply.github.com>

Author: Copilot

Directory.Build.props

@@ -1,5 +1,12 @@
 <Project>
 
+  <!-- Roslyn 4.4 variant output paths must be set here (before Microsoft.Common.props)
+       to avoid MSB3539 warning. See: https://go.microsoft.com/fwlink/?linkid=869650 -->
+  <PropertyGroup Condition="'$(RoslynVersion)' == '4.4'">
+    <BaseOutputPath>$(MSBuildProjectDirectory)/bin/roslyn4.4/</BaseOutputPath>
+    <BaseIntermediateOutputPath>$(MSBuildProjectDirectory)/obj/roslyn4.4/</BaseIntermediateOutputPath>
+  </PropertyGroup>
+
   <PropertyGroup>
     <LangVersion>14</LangVersion>
     <Nullable>enable</Nullable>

src/DocoptNet/DocoptNet.csproj

@@ -5,8 +5,6 @@
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(RoslynVersion)' == '4.4'">
-    <BaseOutputPath>$(_Roslyn44AnalyzerDir)</BaseOutputPath>
-    <BaseIntermediateOutputPath>$(MSBuildProjectDirectory)/obj/roslyn4.4/</BaseIntermediateOutputPath>
     <DefaultItemExcludes>$(DefaultItemExcludes);$(MSBuildProjectDirectory)/obj/**</DefaultItemExcludes>
     <DefineConstants>$(DefineConstants);ROSLYN4_4</DefineConstants>
   </PropertyGroup>