Wrap font-face definitions in @at-root

[bartaz]

Done

Make sure font-face declarations are always on root CSS level

Fixes #4182

QA

• Make sure CI passes and there are no changes in Percy
• Pull the branch locally, and in some local scss file (for example in standalone example) try nesting font declarations:

.test {
  @include vf-b-placeholders;
  @include vf-b-typography;
}

Make sure in the result CSS @font-face doesn't try to use the .test class name.

[webteam-app]

Demo starting at https://vanilla-framework-4190.demos.haus

[bethcollins92]

yay - LGTM