feat(css): add breakpoint margin utility classes

What does this MR do?

This MR adds all the Margin Breakpoint classes to GitLab UI's spacing classes.
Examples: mr-sm-1, mx-lg-4, m-md-3

This was implemented using nested loops so that we don't need to write every class out by hand. If we like this pattern we can refactor the spacing.scss file to do this for all the margin and padding classes.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.