message: fix(theme): add missing prismjs plugin
diff: import { externalLinkIconPlugin } from "@vuepress/plugin-external-link-icon";
  import { nprogressPlugin } from "@vuepress/plugin-nprogress";
  +import { prismjsPlugin } from "@vuepress/plugin-prismjs";
  import { themeDataPlugin } from "@vuepress/plugin-theme-data";
  import { getActiveHeaderLinksPlugin } from "./activeHeaderLinks";
  import { getBlogPlugin } from "./blog";
  @@ -28,6 +29,7 @@ export const getPluginConfig = (
  getActiveHeaderLinksPlugin(plugins.activeHeaderLinks),
  plugins.externalLinkIcon === false ? null : externalLinkIconPlugin(),
  plugins.nprogress === false ? null : nprogressPlugin(),
  + plugins.prismjs === false ? null : prismjsPlugin(),
  themeDataPlugin({ themeData }),
  getBlogPlugin(themeData, plugins.blog),
  getCommentPlugin(plugins.comment),
  