docs: update README.md banner

[Jan-Koll]

Describe the problem

The current banner in README.md still has the old slogan "Cybernetically enhanced web apps".
Also, when viewing README.md (e.g. on the GitHub website/app or a code editor) in a dark-mode, it always only shows a light version of the banner. This gives a weird and inconsistent overall look.

Apparently this also applies to the README.md files in https://github.com/sveltejs/language-tools (and maybe even more repositories that I didn't see yet).

Describe the proposed solution

The banner slogan should be updated to the new slogan "web development for the rest of us".

As for the dark variant, since Markdown supports HTML, it could be added by using a picture element:

<p align="center">
  <a href="https://svelte.dev/">
    <picture>
      <source media="(prefers-color-scheme: dark)" srcset="https://sveltejs.github.io/assets/banner_dark.png"><!--this file doesn't exist. Is there somewhere already a dark variant of the banner? -->
      <img src="https://sveltejs.github.io/assets/banner.png" alt="Svelte - web development for the rest of us" />
    </picture>
  </a>
</p>

If it is not desired to have HTML in the Markdown files, a PNG with a transparent background could also be used.

Importance

nice to have

[Ocean-OS]

This does sound like a good idea (I was actually thinking of making a similar issue/making the image), but I'm not sure if GitHub's flavor of Markdown supports that much HTML— it supports basic syntax like elements, but it gets restrictive with things like CSS, and obviously scripting.

[Jan-Koll]

@Ocean-OS For me it works at least in VSCode and on GitHub. You can test for yourself: https://github.com/Jan-Koll/markdown-picture-test
Note that GitHub settings apparently overwrite the preferred color-scheme of the OS/browser.