src/vs/workbench/contrib/notebook/browser/notebookServiceImpl.ts/NotebookOutputRendererInfoStore/setPreferred
class NotebookOutputRendererInfoStore:	setPreferred(notebookProviderInfo: NotebookProviderInfo, mimeType: string, rendererId: string) {
		const mementoObj = this.preferredMimetype.getValue();
		const forNotebook = mementoObj[notebookProviderInfo.id];
		if (forNotebook) {
			forNotebook[mimeType] = rendererId;
		} else {
			mementoObj[notebookProviderInfo.id] = { [mimeType]: rendererId };
		}

		this.preferredMimetypeMemento.saveMemento();
	}