Add a base font-size variable to calculate type sizes

[lusimeon]

Add a base font-size variable to calculate type sizes rather than use default size (16px)

[titouanmathis]

Some documentation is missing and a condition is missing, seems good to me otherwise :)

[lusimeon]

@titouanmathis If we merge this branch, spaces will be now evaluated based on $font-size-base instead of 16px because of $spaces-base: 8px / 16px * 1rem;.
1rem = $font-size-base instead of 1rem = navigator font size before.

Maybe we should change the $spaces-base to a single value like $spaces-base: 8px; and then in _spaces.scss:

@function space($space) {
  ...
  // Return the value times the base value if our space is a number, or else we
  // return the whole value, allowing us to set `auto` in our `$spaces` list.
  @if (type-of($value) == 'number') {
    @return $value * ($spaces-base / $font-size-base * 1rem);
  } @else {
    @return $value;
  }
}

Unfortunaly, it will create a strong dependency between _typography.scss ($font-size-base) and _spaces.scss ($spaces-base) files…

[titouanmathis]

Unfortunaly, it will create a strong dependency between _typography.scss ($font-size-base) and _spaces.scss ($spaces-base) files…

@lusimeon I think it will be easier to maintain over time if we keep the typography and spaces configuration separate. But the specificity of this configuration should definitely be documented so that people are aware of it.

[titouanmathis]

There is something troubling me in this PR. The ability to set the root font size on which all other sizes are based is definitely a good feature, but this SCSS toolkit CSS output only contains classes and has no side effect on HTML elements. So I think that setting the root font size directly on the html element might not be the best way to set it.

Maybe we should just add a new .type-root-size class that sould be added to the html element on each project.

[titouanmathis]

I am changing the base of this PR from the develop branch to the release/3.0.0 branch in order to include this feature in the future v3.0.0 😉

[titouanmathis]

I merge this PR to send it in the first beta release of the v3.0.0 😉