Add initial support for feature flags

This change adds initial support for feature flags in the
EditorServicesHost so that we can enable new experimental features
conditionally while maintaining current behavior for most users.

Author: daviwil

module/PowerShellEditorServices/PowerShellEditorServices.psm1

@@ -52,6 +52,10 @@ function Start-EditorServicesHost {
         [string]
         $DebugServiceOnly,
 
+        [string[]]
+        [ValidateNotNull()]
+        $FeatureFlags = @(),
+
         [switch]
         $WaitForDebugger
     )
@@ -65,7 +69,8 @@ function Start-EditorServicesHost {
                 $hostDetails,
                 $BundledModulesPath,
                 $EnableConsoleRepl.IsPresent,
-                $WaitForDebugger.IsPresent)
+                $WaitForDebugger.IsPresent,
+                $FeatureFlags)
 
         # Build the profile paths using the root paths of the current $profile variable
         $profilePaths = New-Object Microsoft.PowerShell.EditorServices.Session.ProfilePaths @(

src/PowerShellEditorServices.Host/EditorServicesHost.cs

@@ -8,6 +8,7 @@
 using Microsoft.PowerShell.EditorServices.Session;
 using Microsoft.PowerShell.EditorServices.Utility;
 using System;
+using System.Collections.Generic;
 using System.Diagnostics;
 using System.Reflection;
 using System.Threading;
@@ -33,6 +34,7 @@ public class EditorServicesHost
         private HostDetails hostDetails;
         private string bundledModulesPath;
         private DebugAdapter debugAdapter;
+        private HashSet<string> featureFlags;
         private LanguageServer languageServer;
 
         #endregion
@@ -60,13 +62,15 @@ public EditorServicesHost(
             HostDetails hostDetails,
             string bundledModulesPath,
             bool enableConsoleRepl,
-            bool waitForDebugger)
+            bool waitForDebugger,
+            string[] featureFlags)
         {
             Validate.IsNotNull(nameof(hostDetails), hostDetails);
 
             this.hostDetails = hostDetails;
             this.enableConsoleRepl = enableConsoleRepl;
             this.bundledModulesPath = bundledModulesPath;
+            this.featureFlags = new HashSet<string>(featureFlags ?? new string[0]);
 
 #if DEBUG
             int waitsRemaining = 10;