(PowerShellGH-1336) Hide syntax aware folding behind a feature flag

Previously the syntax folding was available for all users.  However as this is
a new feautre which depends on the grammar file which has been unreliable, it
was decided to hide this feature behind a flag.  The commit adds the logic to
only active the syntax folding if the "syntax-folding" feature flag is set in
the "powershell.developer.featureFlags" setting.

Author: glennsarti

src/features/Folding.ts

@@ -6,6 +6,7 @@ import {
 } from "vscode-languageclient";
 import { IFeature } from "../feature";
 import { Logger } from "../logging";
+import * as Settings from "../settings";
 
 /**
  * Defines a grammar file that is in a VS Code Extension
@@ -498,6 +499,12 @@ export class FoldingFeature implements IFeature {
      * @param documentSelector documentSelector object for this Folding Provider
      */
     constructor(private logger: Logger, documentSelector: DocumentSelector) {
+        // Hide the syntax folding behind a feature flag
+        const settings = Settings.load();
+        if (settings.developer && settings.developer.featureFlags) {
+            if (settings.developer.featureFlags.indexOf("syntax-folding") === -1) { return; }
+        }
+
         const grammar: IGrammar = this.grammar(logger);
 
         // If the PowerShell grammar is not available for some reason, don't register a folding provider,