feat: add responsive design (fix #33)

[hugoattal]

Linked to: #33

Description

It's not 100% done yet, but I feel like it's a good first step (and that the PR is long enough)
It's missing controls, for now you can just select a file and a variant.

I'll add the control panel in another PR

Additional context

You can select story/variant with a menu available at the top of the screen:

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[Akryum]

Why did you rename the files to scss? We don't use Sass, we only use PostCSS. 🐈
If you want you can rename them to .postcss or .pcss

[hugoattal]

The content of those files wasn't "pure" CSS (and I thought it was SASS?), here are a few examples:

.htw-prose {
  h1, h2, h3, h4 {
    &:not(:hover) {
      .header-anchor[aria-hidden="true"] {
        visibility: hidden;
      }
    }
  }
}

or

.__histoire-scale-x {
  &-enter-active,
  &-leave-active {
    transition: transform .15s, opacity .15s;
  }

  &-enter-from,
  &-leave-to {
    transform: scaleX(0);
    opacity: 0;
  }
}

So my IDE complained a lot and formatting was impossible...

EDIT: ooooh, ok, I'll rename in pcss then!