Allow for alternate versions of documents to be semantically highlighted

[JoeRobich]

To fix diff highlighting in the VSCode C# extension, we need to pass along alternate versions of the document we are highlighting. Request has a Buffer property but populating that will invoke the updateBufferHandler when the request is handled (as alluded to in dotnet/vscode-csharp#4915 (comment)).

omnisharp-roslyn/src/OmniSharp.Host/Endpoint/EndpointHandler.cs

Lines 109 to 116 in 52f9ed3

	if (request is Request && _updateBufferHandler.Value != null)
	{
	var realRequest = request as Request;
	if (!string.IsNullOrWhiteSpace(realRequest.FileName) && (realRequest.Buffer != null || realRequest.Changes != null))
	{
	await _updateBufferHandler.Value.Process(packet, model, requestObject);
	}
	}

This PR adds an optional VersionedText property to the highlight request which can carry the alternate version of the document contents. The highlight service will then swap the document text for the duration of the request when it is populated.

[filipw]

thanks! 👏