Integrate with FirefoxCSS store

[gwennlbh]

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

The registry for FirefoxCSS Store is located in a JSON file, 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)
2. [ ] checks if slug.Slug(theme["title"]) + ".yaml" is already in the registry.
3. [ ] 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
4. [ ] 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 to talk about it: An optional ffcss_name entry in the JSON could be added to show sth. like

$ ffcss use <ffcss_name>    (?)

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”)

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