Support for :global() rules

[turadg]

Something like this within a component:

const globals = css`
  :global() {
    html {
      font-size: large;
    }
  }
`;
errors with:

Error: Syntax error: Selector "html" is not pure (pure selectors must contain at least one local class or id)

It works okay if it's in an `_app.tsx` file.

Known issue in https://github.com/callstack/linaria/issues/589#issuecomment-730219118

Documenting here for anyone coming to this project.