Use LspCompletionItemKind.Variable for parameters in the completion provider

[333fred]

https://github.com/dotnet/roslyn/blob/master/src/Features/LanguageServer/Protocol/Extensions/ProtocolConversions.cs#L57

We map parameters to values today, when they should likely be considered variables instead. See also OmniSharp/omnisharp-roslyn#2061.

/cc @allisonchou @dibarbet.