Add support for paged media with @page at-rule

[prestancedesign]

Hi,

I contribute with this PR to add @page at-rule syntax:
https://developer.mozilla.org/en-US/docs/Web/CSS/@page

Firstly, this adds support for this specific case:

@page {
  /* margin box at top right showing page number */
  @top-right {
    content: "Page " counter(pageNumber);
  }
}

If interested, I'll also add the support of pseudo-classes.

/* Targets all even-numbered pages */
@page :left {
  margin-top: 4in;
}

Best regards.
Mike