build(deps-dev): bump @vuepress/plugin-search (#985)

Bumps [@vuepress/plugin-search](https://github.com/vuejs/vuepress/tree/HEAD/packages/@vuepress/plugin-search) from 2.0.0-beta.49 to 2.0.0-beta.51.
- [Release notes](https://github.com/vuejs/vuepress/releases)
- [Changelog](https://github.com/vuejs/vuepress/blob/master/CHANGELOG.md)
- [Commits](https://github.com/vuejs/vuepress/commits/HEAD/packages/@vuepress/plugin-search)

---
updated-dependencies:
- dependency-name: "@vuepress/plugin-search"
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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