feat: refactor vite styles plugin

[userquin]

This PR includes:

• simplify virtual logic: ~~refactor logic to remove any virtual usage: using target mapping from .css to .sass + writing temporary SASS files when using SASS Variables (will solve missing sass source warnings and SSR problems when using Nuxt)~~ ✔️ we only need to fix vue router warnings when running Nuxt with SSR enabled
• handle none and object styles notation at once
• include useViteFileImport in shared options
• docs: include similar hint to Vuetify docs in readme file

You can check the following repos and the corresponding PR:

• https://github.com/userquin/nuxt3-vuetify3-issue-15412/pull/3 (Nuxt)
• https://github.com/userquin/vuetify-vite-modern-sass/pull/3 (Vite)

supersedes #332