Is your feature request related to a problem? Please describe.
I tried to make a plugin to support asciidoctor files. I can easily transform adoc-file to html, but then I need to do a bunch of things over this html string to make all images correctly included into bundle (find all img tags and theirs src, transform them to imports, update src, etc). In webpack this is covered by html-loader. I see that this is also happened in plugin vite:build-html

Describe the solution you'd like
Maybe you can export API used in this plugin?

Describe alternatives you've considered
Without this the only way I see is too copy a bunch of code from vite or html-loader internals