electron - explicitly disable spellchecker

It will be enabled by default starting with Electron 9

src/vs/code/electron-main/auth.ts

@@ -64,6 +64,7 @@ export class ProxyAuthHandler extends Disposable {
 				sandbox: true,
 				devTools: false,
 				enableRemoteModule: false,
+				spellcheck: false,
 				v8CacheOptions: 'bypassHeatCheck'
 			}
 		};