Settings sync: before shutdown cannot be async in web

[bpasero]

A reminder that in browser long running shutdown handlers will not work:

vscode/src/vs/workbench/services/userDataSync/browser/userDataSyncWorkbenchService.ts

Lines 286 to 298 in d014eb1

	private async onBeforeShutdown(manualSyncTask: IManualSyncTask): Promise<boolean> {
	const result = await this.dialogService.confirm({
	type: 'warning',
	message: localize('sync in progress', "Settings Sync is being turned on. Would you like to cancel it?"),
	title: localize('settings sync', "Settings Sync"),
	primaryButton: localize('yes', "Yes"),
	secondaryButton: localize('no', "No"),
	});
	if (result.confirmed) {
	await manualSyncTask.stop();
	}
	return !result.confirmed;
	}

I suggest to move this into a electron-sandbox namespace to be clear.

[sandy081]

I cannot move just this to electron-sandbox, instead I will add web check and add a note that it works only in desktop.