Warn with proper guidance for trying to use Vue without the plugin installed

[antfu]

Is your feature request related to a problem? Please describe.
#1365

Currently, we are assuming Vue file as JSX without @vitejs/plugin-vue. This is potentially confusing.

Describe the solution you'd like
Show the warning and guidance for users to install Vue's plugin

[yyx990803]

Most users should create a new Vite 2 app with @vitejs/create-app, and will be prompted to select proper templates, so they won't actually run into this problem. This only happens to people who don't read the docs.