docs: update README banner (#15063)

* fix

* update banner image

this is an empty commit whose name will be shown in the commit history (if someone looks at the commit history they should be able to understand what commit does what).

* chore: fix spacing

* chore: fix spacing

---------

Co-authored-by: Paolo Ricciuti <ricciutipaolo@gmail.com>

README.md

@@ -1,4 +1,9 @@
-[![Cybernetically enhanced web apps: Svelte](https://sveltejs.github.io/assets/banner.png)](https://svelte.dev)
+<a href="https://svelte.dev">
+	<picture>
+		<source media="(prefers-color-scheme: dark)" srcset="assets/banner_dark.png">
+		<img src="assets/banner.png" alt="Svelte - web development for the rest of us" />
+	</picture>
+</a>
 
 [![license](https://img.shields.io/npm/l/svelte.svg)](LICENSE.md) [![Chat](https://img.shields.io/discord/457912077277855764?label=chat&logo=discord)](https://svelte.dev/chat)
 

packages/svelte/README.md

@@ -1,4 +1,9 @@
-[![Cybernetically enhanced web apps: Svelte](https://sveltejs.github.io/assets/banner.png)](https://svelte.dev)
+<a href="https://svelte.dev">
+	<picture>
+		<source media="(prefers-color-scheme: dark)" srcset="../../assets/banner_dark.png">
+		<img src="../../assets/banner.png" alt="Svelte - web development for the rest of us" />
+	</picture>
+</a>
 
 [![npm version](https://img.shields.io/npm/v/svelte.svg)](https://www.npmjs.com/package/svelte) [![license](https://img.shields.io/npm/l/svelte.svg)](LICENSE.md) [![Chat](https://img.shields.io/discord/457912077277855764?label=chat&logo=discord)](https://svelte.dev/chat)