This stylelint-scss rule is useful because we were inconsistent when
using calc(); sometimes we added interpolation to Sass variables, and
sometimes we didn't. The reason why we originally added interpolation
was that it was necessary until we migrated to Dart Sass in commit
d54971e53. Since then, we can omit the interpolation, which is also what
the Sass documentation recommends [1].
[1] https://sass-lang.com/documentation/values/calculations/
In the previous commit, we used the `calc` function when assiging CSS
properties in order to avoid warnings like:
```
Deprecation Warning: Using / for division outside of calc() is
deprecated and will be removed in Dart Sass 2.0.0.
Recommendation: math.div($global-width, 2) or calc($global-width / 2)
```
In cases like dividing by two, there's a third alternative: multiplying
by 0.5. We're applying this principle to all variable assignments where
we were using divisions, since using the `calc` function here would
sometimes result in errors due to these variables being used in
arithmetical operations. We aren't using `math.div` because it makes the
code harder to read.
The division operator `/` from Sass is deprecated because `/` is used in
CSS for uses other than dividing numbers. That's why we were getting
many warnings like:
```
Deprecation Warning: Using / for division outside of calc() is
deprecated and will be removed in Dart Sass 2.0.0.
Recommendation: math.div($line-height, 2) or calc($line-height / 2)
More info and automated migrator: https://sass-lang.com/d/slash-div
margin-top: $line-height / 2;
```
Since using math.div makes the code harder to read and `calc` is
universally supported by all browsers (although the implementation in
Internet Explorer doesn't work in certain cases), we're using `calc`
when assigning the value to a CSS property.
However, we're also using divisions when assigning Sass variables, and
in those cases using `calc` is trickier because sometimes these
variables are used in other operations. We'll handle these cases in the
next commit.
