Integrate with FirefoxCSS store

URL: https://firefoxcss-store.github.io/

The registry for FirefoxCSS Store is located in [a JSON file, `themes.json`](
https://raw.githubusercontent.com/FirefoxCSS-Store/FirefoxCSS-Store.github.io/main/themes.json)

Maybe add a theme browser in the CLI that pulls from that?

Either way, add a command `ffcss pull` that:

1. [ ] reads themes from that JSON file (using the URL)
1. [ ] checks if `slug.Slug(theme["title"]) + ".yaml"` is already in the registry.
1. [ ] if it isn't:
1. [ ] `ffcss use` it with `theme["link"]` _Note: links that have `/tree/master/`... need to be handled: if there's multiple themes with links that only differ after `/tree/`, it means that they are variants of each other_
1. [ ] If it is: warn the user that a theme wasn't added because it conflicts with an installed theme (and name it)

Maybe contact [Neikon](https://github.com/Neiko) to talk about it: An optional `ffcss_name` entry in the JSON could be added to show sth. like

```
$ ffcss use (?)
```

below the themes' cards

(and when you hover over the `(?)` icon, it tells sth like “This theme can be installed on the command line using [ffcss](https://github.com/ewen-lbh/ffcss)”)

- [ ] Make a PR for that!
- [ ] Parse `\nCompatibility: V89+` in `theme["description"]` into a FirefoxVersionConstraint