Add workspace and document symbol capabilities

This change adds two new fields to the ServerCapabilities class:
WorkspaceSymbolProvider and DocumentSymbolProvider.  Setting these to true
causes the VS Code extension to send requests for workspace and document
symbols when the user triggers the corresponding actions.

Author: daviwil

src/PowerShellEditorServices.Host/LanguageServer.cs

@@ -100,6 +100,8 @@ await requestContext.SendResult(
                         DefinitionProvider = true,
                         ReferencesProvider = true,
                         DocumentHighlightProvider = true,
+                        DocumentSymbolProvider = true,
+                        WorkspaceSymbolProvider = true,
                         HoverProvider = true,
                         CompletionProvider = new CompletionOptions
                         {

src/PowerShellEditorServices.Protocol/LanguageServer/ServerCapabilities.cs

@@ -27,6 +27,10 @@ public class ServerCapabilities
         public bool? ReferencesProvider { get; set; }
 
         public bool? DocumentHighlightProvider { get; set; }
+
+        public bool? DocumentSymbolProvider { get; set; }
+
+        public bool? WorkspaceSymbolProvider { get; set; }
     }
 
     /// <summary>