Switched to Microsoft.VisualStudio.Language v17.6.268 - Autocomplete …

…compatibility fix part 1 (#198)

Autocomplete compatibility fix allowing to support autocomplete starting
from Visual Studio VS 17.6+
It was tested against VS versions:
 - 17.7.6
 - 17.9 Preview 2
 - 17.10.3
 - 17.12
 - 17.13.0 Preview 2.1

Internally `Microsoft.VisualStudio.Language` version is set to 17.0.0.0 after compilation, regardless of `Microsoft.VisualStudio.Language` version used as nuget package. The important part is `Microsoft.VisualStudio.Threading` library, which version set explicitly to match nuget `Microsoft.VisualStudio.Language` version.

src/Cody.VisualStudio/Cody.VisualStudio.csproj

@@ -125,7 +125,7 @@
   </ItemGroup>
   <ItemGroup>
     <PackageReference Include="Microsoft.VisualStudio.Language">
-      <Version>17.11.260</Version>
+      <Version>17.6.268</Version>
     </PackageReference>
     <PackageReference Include="Microsoft.VisualStudio.SDK" Version="17.0.0-previews-4-31709-430" ExcludeAssets="runtime" />
     <PackageReference Include="Microsoft.VSSDK.BuildTools" Version="17.6.2164">