chore(deps-dev): bump vuepress from 1.7.1 to 1.8.0

Bumps [vuepress](https://github.com/vuejs/vuepress/tree/HEAD/packages/vuepress) from 1.7.1 to 1.8.0.
- [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/v1.8.0/packages/vuepress)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]