Need API for enumerating AnalyzerConfigOptions

[JoeRobich]

Since the naming style rules are not tied to a set of known editorconfig options, The NamingStylePreferenceEditorConfigStorageLocation expects to be able to enumerate through the keys of the options to deserialize the preference. The AnalyzerConfigOptions API does not provide a way to perform this enumeration.

My proposal is that we add getter for accessing the Keys as an IEnumerable<string>

public abstract class AnalyzerConfigOptions
{
    ...
    /// <summary>
    /// Enumerates keys of all available options in no specific order.
    /// </summary>
    /// <exception cref="NotImplementedException">Not implemented by the derived type.</exception>
    public virtual IEnumerable<string> Keys
         => throw new NotImplementedException();
    ...
}

The property is virtual to avoid a breaking change.