feat: implementing the base @scope

[yisibl]

This PR adds support for the base @scope.

See also:

• https://chromium-review.googlesource.com/c/chromium/src/+/4161741
• Explainer: https://css.oddbird.net/scope/explainer/#:~:text=finally%2C%20we%20could%20allow%20%40scope%20without%20any%20selector%20clauses

@scope {
  .foo {
    display: flex;
  }
}

[devongovett]

Do you think we should put this behind a flag like we have for nesting since the spec is still draft and potentially changing? Also since it is not fully implemented yet.

[yisibl]

Ok, I've added a flag to disable it by default.

But I can't run yarn website:build locally, can you take a look?