Fix lang service reporting C# 7.3 errors in Error List

By setting LangVersion so late in msbuild evaluation, the `CommandLineArgsForDesignTimeEvaluation` property is set based on the default value (of 7.3, for .NET Framework projects). This causes the VS language service to initialize with the wrong language version.

I originally moved this property definition to the .targets because `$(Language)` isn't set in time for the .props files to use them. But we can move it back to .props by using the project file extension instead.

Author: AArnott

Directory.Build.props

@@ -40,6 +40,11 @@
     <SymbolPackageFormat>snupkg</SymbolPackageFormat>
   </PropertyGroup>
 
+  <PropertyGroup>
+    <LangVersion Condition="'$(MSBuildProjectExtension)'=='.csproj'">13</LangVersion>
+    <LangVersion Condition="'$(MSBuildProjectExtension)'=='.vbproj'">16.9</LangVersion>
+  </PropertyGroup>
+
   <ItemGroup>
     <AdditionalFiles Include="$(MSBuildThisFileDirectory)stylecop.json" Link="stylecop.json" />
   </ItemGroup>

Directory.Build.targets

@@ -1,9 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project>
-  <PropertyGroup>
-    <LangVersion Condition="'$(Language)'=='C#'">13</LangVersion>
-    <LangVersion Condition="'$(Language)'=='VB'">16.9</LangVersion>
-  </PropertyGroup>
   <ItemGroup>
     <!-- Avoid compile error about missing namespace when combining ImplicitUsings with .NET Framework target frameworks. -->
     <Using Remove="System.Net.Http" Condition="'$(TargetFrameworkIdentifier)'=='.NETFramework'" />