[css-variables] support the var() function in @page blocks

[ondras]

According to the discussion at https://bugs.chromium.org/p/chromium/issues/detail?id=763298, the var() function is not specced to work inside @page at-rule. However, using custom properties inside these blocks might be a reasonable requirement:

:root {
  --margin: 3cm;
  --width: 1024;
  --height: 768;
}

@page {
  margin: var(--margin);
  size: calc(var(--width) * 1px) calc(var(--height) * 1px)
}

Relevant spec: https://drafts.csswg.org/css-variables/#funcdef-var

[upsuper]

env() should serve this use case instead of var().

[tabatkins]

Right, page boxes don't inherit from anything, so there's no way for them to receive a custom property value. Theoretically we could define that they inherit custom props from the root, but env() is indeed better suited for this use-case.