Resolve html entry points with plugins

[remorses]

Is your feature request related to a problem? Please describe.
Currently the build and prebundle steps use globs to find entry points, this is not optimal because globs can be slow on large folders and there is no way to pass virtual html files to vite

Describe the solution you'd like
Use the resolveId and load hooks to resolve html entry files, this would let users load virtual entry points without relying on a glob search

If no entry points are found using plugins, fallback to using a glob search

Frameworks built on top of vite wouldn't need to write a temporary html file and would use a virtual html file instead

Describe alternatives you've considered

Additional context

[IndexXuan]

this will help vue-cli migrations to vitie more smoothly ~ vite do need html for serve but it is nice to have virtual html entry.

[IndexXuan]

https://github.com/IndexXuan/vite-plugin-html-template
It works for me. like html-webpack-plugin for vite.