Add an option to specify the location of rollup's dev entry point

[webJose]

          I'll leave some notes here as I work through the experiment.

We could provide an option to specify the location to the index.html file, since SvelteKit's default location is src/app.html.

Originally posted by @davidhaley in https://github.com/WJSoftware/vite-plugin-single-spa/issues/1#issuecomment-1688044239