Auto converting let -> const where possible in mainthread and extHost

Const provides better type guards and can make logic clearer

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

@@ -154,7 +154,7 @@ export class ExtHostCommands implements ExtHostCommandsShape {
 		}
 
 		try {
-			let result = callback.apply(thisArg, args);
+			const result = callback.apply(thisArg, args);
 			return Promise.resolve(result);
 		} catch (err) {
 			this._logService.error(err, id);