Add initial support for feature flags

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

This introduces a new setting:

`powershell.developer.featureFlags`

Author: daviwil

package.json

@@ -325,6 +325,11 @@
           "default": "",
           "description": "Specifies the path to a PowerShell Script Analyzer settings file. Use either an absolute path (to override the default settings for all projects) or use a path relative to your workspace."
         },
+        "powershell.developer.featureFlags": {
+          "type": "array",
+          "default": [],
+          "description": "An array of strings used to enable experimental features in the PowerShell extension."
+        },
         "powershell.developer.powerShellExePath": {
           "type": "string",
           "default": "",

scripts/Start-EditorServices.ps1

@@ -58,6 +58,9 @@ param(
     [string]
     $DebugServiceOnly,
 
+    [string[]]
+    $FeatureFlags,
+
     [switch]
     $WaitForDebugger,
 

src/session.ts

@@ -266,9 +266,15 @@ export class SessionManager {
 
             var editorServicesLogPath = this.log.getLogFilePath("EditorServices");
 
+            var featureFlags =
+                this.sessionSettings.developer.featureFlags !== undefined
+                ? this.sessionSettings.developer.featureFlags.map(f => `'${f}'`).join(', ')
+                : "";
+
             startArgs +=
-                "-LogPath '" + editorServicesLogPath + "' " +
-                "-SessionDetailsPath '" + utils.getSessionFilePath() + "' ";
+                `-LogPath '${editorServicesLogPath}' ` +
+                `-SessionDetailsPath '${utils.getSessionFilePath()}' ` +
+                `-FeatureFlags @(${featureFlags})`
 
             var powerShellArgs = [
                 "-NoProfile",

src/settings.ts

@@ -23,6 +23,7 @@ export interface IScriptAnalysisSettings {
 }
 
 export interface IDeveloperSettings {
+    featureFlags?: string[];
     powerShellExePath?: string;
     bundledModulesPath?: string;
     editorServicesLogLevel?: string;
@@ -47,6 +48,7 @@ export function load(myPluginId: string): ISettings {
     };
 
     let defaultDeveloperSettings: IDeveloperSettings = {
+        featureFlags: [],
         powerShellExePath: undefined,
         bundledModulesPath: undefined,
         editorServicesLogLevel: "Normal",