feat(vuepress): add more exports and bin shorthands (close #1446)

[Mister-Hope]

We can also add vuepress-vite and vuepress-webpack bin for vuepress package, not sure about this.

[meteorlxy]

My thoughts:

• We can use create helper to help scaffolding a new project
• May not need an extra --bundler option. We don't need to automatically resolving bundlers