Add vuepress detector

[shortdiv]

- Summary
Running netlify dev doesn't work for vuepress projects. This fix adds a detector so that the dev server can run.

- Test plan

Add vuepress to your project

# install as a local dependency
yarn add -D vuepress # OR npm install -D vuepress

# create a docs directory
mkdir docs
# create a markdown file
echo '# Hello VuePress' > docs/README.md

Then update your package.json file so it has the proper command for running vuepress.

{
  "scripts": {
    "dev": "yarn run docs:dev",
  }
}

Try running netlify dev. You should have a netlify dev server accessible at localhost:8888

- Description for the changelog
Adds Vuepress to the list of detectors

- A picture of a cute animal

[lunaceee]

LGTM! ✨