Transform for index.html

[antfu]

Is your feature request related to a problem? Please describe.
I am trying to make a PWA plugin for Vite while I need to inject some tags into index.html. However, the transforms api does not seem to be able to capture it.

Describe the solution you'd like
Pass through index.html into transforms

Describe alternatives you've considered

Additional context

[underfin]

Track here #528 :)