Add dark mode version of hero image

[JHWelch]

Adds a Dark Theme version of the hero image for the README

Colors copied from Dark version of Vite PWA Website

BEFORE

Light

Dark

AFTER

Light

Dark

[userquin]

We've both svg (in figma not in the repo) and so we can add logic I added here: https://github.com/userquin/vuetify-nuxt-module/blob/main/README.md

<div align="center">
<picture>
  <source media="(prefers-color-scheme: dark)" srcset="https://github.com/userquin/vuetify-nuxt-module/raw/main/hero-dark.svg" />
  <img alt="vuetify-nuxt-module - Zero-config Nuxt Module for Vuetify" src='https://github.com/userquin/vuetify-nuxt-module/raw/main/hero.svg' alt="vuetify-nuxt-module - Zero-config Nuxt Module for Vuetify"><br>
</picture>
<p>Zero-config Nuxt Module for Vuetify</p>
</div>