message: fix(feed2): make sure parent dir exists
diff: import { chalk, fs } from "@vuepress/utils";
  +import { dirname } from "path";
  import { Feed } from "./feed";
  import { FeedPage } from "./page";
  import { getFilename, logger } from "./utils";
  @@ -50,6 +51,7 @@ export class FeedGenerator {
  if (this.options.atom) {
  logger.load("Generating Atom Feed");
  + await fs.ensureDir(dirname(dest(atomOutputFilename)));
  await fs.outputFile(dest(atomOutputFilename), this.feed.atom());
  logger.update(
  @@ -64,6 +66,7 @@ export class FeedGenerator {
  if (this.options.json) {
  logger.load("Generating JSON Feed");
  + await fs.ensureDir(dirname(dest(jsonOutputFilename)));
  await fs.outputFile(dest(jsonOutputFilename), this.feed.json());
  logger.update(
  @@ -78,6 +81,7 @@ export class FeedGenerator {
  if (this.options.rss) {
  logger.load("Generating RSS Feed");
  + await fs.ensureDir(dirname(dest(rssOutputFilename)));
  await fs.outputFile(dest(rssOutputFilename), this.feed.rss());
  logger.update(
  