improve integration for custom file extensions with vite

[dominikg]

Describe the feature

Currently vite only knows that .svelte is transformed to js but svelte can be used with other extensions aswell (eg .svx , .svg )

1. enhance vite to allow configuration of "extensions that transform to js"
2. configure vite from vite-plugin-svelte with the extensions of svelte config.

A Usecase
Resolve the current situation where non ".svelte" svelte components require ?import to be imported correctly

Alternatives
Check out if the import marker could be added during resolveId?

Additional context
?import marker added in sveltekit: https://github.com/sveltejs/kit/blob/1c78dec3b65d407db70a555ff54cc11442dcdc07/packages/kit/src/core/dev/index.js#L254

initial PR on vite (needs to be rebased)
vitejs/vite#3219

new PR on vite
vitejs/vite#3828

[dominikg]

unfortunately vitejs/vite#3828 has been set to Draft and it is unclear how vite is handling this in the future. So for now we wait...

[dominikg]

closing for now, the issue is going to have to be tackled from the vite side first and then we can update vps to consume the changes on the vite side for extension handling