Consider subset match valid for symbol lookups and completions

[filipw]

Given the following code:

    public class Options { }
    public interface IConfigurationOptions { }
    public class ConfigurationOptions : IConfigurationOptions { }

When passing Opt into symbol lookup or completion service, we will only return Options, which is not the optimal user experience.
In fact, this is not in line with the Visual Studio behavior, where in both symbol lookup and completion, opt returns all 3 possibilities.

This PR changes a subset match of length longer than 1 to be valid when verifying symbol/completion subsequences, resulting in much better experience.

[DustinCampbell]

Looks good!

Someday, we'll just want to do all of the matching we do in Roslyn, but this is simple enough to tweak and improve now.

[david-driscoll]

I'd love to add better fuzzy searching (Resharper like) but today this makes it better.