Support pug as entry file

[guillenotfound]

Is your feature request related to a problem? Please describe.
Imagine that you have several UIs depending on an environment variable, it's possible in JS land, but how would you do it in plain HTML?

VITE_UI=example1 npm run dev
VITE_UI=example2 npm run dev

Describe the solution you'd like
Support pug or any other templating language as entrypoint to allow sharing an index.pug and changing stuff like title, fonts, analytics,...

Describe alternatives you've considered
Mustache or jinja, but pug is already supported, which makes it very convenient.

[antfu]

With #793 & custom index.html path, you could probably make a plugin for that 😉

[yyx990803]

This is out of scope for Vite.

Actually, in 2.0 you can use a plugin + transformIndexHtml hook to do this.

[marlonmarcello]

Unfortunately I don't think #793 solves this issue.
Even by making a plugin using transformIndexHtml you would still need an index.html.

You can't have a structure like this for example:

_main.pug
index.pug << extends _main.pug
about/
  |-- index.pug << extends _main.pug
vite.config.js

I've been trying to develop a Pug plugin and I got stuck.
I also made a demo repo if that helps.
Any help is much appreciated