Added spacing utility classes

Implemented classes for the spacing sizes (in pixels) in our design system.
Provided utility classes for margins
- m - margin
- mx - margin-left, margin-right
- my - margin-top, margin-bottom
- mt - margin-top
- mr - margin-right
- mb - margin-bottom
- ml - margin-left
Provided utility classes for paddings
- p - padding
- px - padding-left, padding-right
- py - padding-top, padding-bottom
- pt - padding-top
- pr - padding-right
- pb - padding-bottom
- pl - padding-left
  Example of usage:
```
className="m-1-5 px-3 py-0-5"
```
Created mixin generate-utilities that generates utility classes based on a provided configuration map. Our utility classes can be easily scaled with this mixin

Reorganized creation of CSS variables by replacing generate-css-variables($map, $prefix) with register-vars($vars, $name)
Created get-var($name, $suffix) function that retrieves the value of a CSS variable by constructing its name using a base name and optional suffix.
These changes will reduce the amount of boilerplate code when working with CSS variables.

ita-social-projects / SpaceToStudy-Client