Make VIRTUAL_PREFIX configurable

[mubaidr]

Chrome extensions does not support files or directory with __ or _ prefix. So, this library is unusable for development of extensions.

I am also not sure why this issue does not occur for build but only happens during dev mode.
 
Reproduction: https://github.com/mubaidr/vite-vue3-browser-extension-v3/tree/cs-sd

Just clone, install and then run dev. This will generate __vue-router/auto-routes in output. Rendering unusable for chrome extension devolvement.

Related discussion: #452