[Feature] index.html / dev server proxy

[jpmaga]

Is your feature request related to a problem? Please describe.
Not really a problem, more like a limitation from what I could find in the documentation, and a stroll through the code. I might be jumping the gun a bit, but I seem to be unable to use this outside the defined structure. I don't seem to be able to run it from other places.

Example:
server on port 9000 only has an index.html file, with the script src as http://localhost:3000/something.js (vite dev server), the first error it will get is about cors. Setting it in vite.config.js doesn't work, probably some order hierarchy issue, If i go to vite's dist directory in node_modules and add it after creating the Koa app, this error goes away.

const cors = require('@koa/cors')
app.use(cors({ origin: '*' }))

Now, the second error, is that it is trying to access files under [http://localhost:9000]/index.css for example, instead of localhost:3000, also the websocket is ws://localhost:9000 instead of the proper vite's dev server: ws://localhost:3000

Describe the solution you'd like
While developing, I think there should be a way to to proxy the index.html content, as this would allow vite to be used in a lot more contexts, for example with web server serving php files, like some other tools do (webpack, browsersync, etc).

Describe alternatives you've considered
I haven't considered any. Just trying a major step up in speed from webpack.

Additional context
Personally, this request primarily comes from the fact that I have a lot of laravel projects and would be nice to be able to use vite for them.

Cheers, and congrats on the amazing job on this.

[yyx990803]

Thanks - tracking in #147