(GH-412) Strongly type ProtocolType in settings

This changes the settings to strongly type `ProtocolType` instead of
using `string` and forcing downstream code to parse it correctly. It
instead is cast when settings is used, and this does not fail because
it is a typed array inside package.json.

Author: jpogran

src/settings.ts

@@ -18,7 +18,7 @@ export interface IEditorServiceSettings {
   enable?: boolean;
   featureflags?: string[];
   loglevel?: string;
-  protocol?: string;
+  protocol?: ProtocolType;
   tcp?: IEditorServiceTCPSettings;
   timeout?: number;
 }