Continue fixing TS 2.3.1 Compiler Errors in VSCode Codebase (#25315)

From: https://github.com/Microsoft/TypeScript/issues/15352

TS 2.3.1 introduced a breaking change by [switching to covariant types for callbacks](https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#covariance-in-callback-parameters). This change tries to fix these compiler errors in the workbench codebase

src/vs/workbench/api/node/extHost.api.impl.ts

@@ -191,7 +191,7 @@ export function createApiFactory(
 			}
 
 			executeCommand<T>(id: string, ...args: any[]): Thenable<T> {
-				return extHostCommands.executeCommand(id, ...args);
+				return extHostCommands.executeCommand<T>(id, ...args);
 			}
 
 			getCommands(filterInternal: boolean = false): Thenable<string[]> {

src/vs/workbench/api/node/extHostQuickOpen.ts

@@ -91,7 +91,7 @@ export class ExtHostQuickOpen extends ExtHostQuickOpenShape {
 				return TPromise.wrapError(err);
 			});
 		});
-		return wireCancellationToken(token, promise, true);
+		return wireCancellationToken<Item>(token, promise, true);
 	}
 
 	$onItemSelected(handle: number): void {

src/vs/workbench/api/node/mainThreadCommands.ts

@@ -43,7 +43,7 @@ export class MainThreadCommands extends MainThreadCommandsShape {
 	}
 
 	$executeCommand<T>(id: string, args: any[]): Thenable<T> {
-		return this._commandService.executeCommand(id, ...args);
+		return this._commandService.executeCommand<T>(id, ...args);
 	}
 
 	$getCommands(): Thenable<string[]> {