Add target: variant

[neupauer]

Hey 👋!

This PR adds target: variant for the CSS selector :target.

---

Why?

A lot of sites has fixed nav on top of the page. When you navigate to an anchor link it gets covered up by a fixed header.

With target: variant we can target active anchor on the page. Step two is to add an offset, what is possieble by adding e.g., scroll-margin-top: 20px. When this PR would be accepted I would like to create also PR for the mentioned scroll-margin-* utilities.

<nav class="fixed h-20">
  <a href="#hello">Hello</a>
</nav>

<section id="hello" class="target:scroll-margin-top-24">
  Hello World
</section>

Demo: https://yie6f.csb.app/#target-2
Demo Code: https://codesandbox.io/s/scroll-margin-yie6f?file=/index.html

[adamwathan]

Hey thanks for this! I've collapsed this feature into #4482 and added you as a co-author on it to make sure you get credit for it but going to close this PR in favor of that one ❤️