(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

@@ -5,6 +5,7 @@ import {
     LanguageClient,
 } from "vscode-languageclient";
 import { IFeature } from "../feature";
+import * as Settings from "../settings";
 
 const tm = getCoreNodeModule("vscode-textmate");
 // Returns a node module installed with VSCode, or null if it fails.
@@ -394,6 +395,12 @@ export class FoldingFeature implements IFeature {
     private foldingProvider: FoldingProvider;
 
     constructor(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; }
+        }
+
         this.foldingProvider = new FoldingProvider();
         vscode.languages.registerFoldingRangeProvider(
             documentSelector,