Support :root pseudo-class selector and rem unit as expected

[yhatt]

Recognize :root pseudo-class selector and rem unit and transform styles while rendering, to reflect the view that the author expected by CSS theme.

• :root will be transformed into section.
• font-size declarations containing in the rule targeted into section (includes transformed from :root) will assign to section container as --marpit-root-font-size CSS custom property.
• XXrem unit in all CSS declarations will be transformed into calc(var(--marpit-root-font-size, 1rem) * XX). Thus, rem within the context of Marpit will always be indicated a relative size from the root element of the slide (<section>).

:root {
  font-size: 16px;
}
h1 {
  font-size: 2rem;
}

The above theme CSS will transform into this CSS in the default constructor options:

div.marpit > section {
  font-size: 16px;
}
div.marpit > section {
  --marpit-root-font-size: 16px;
}
div.marpit > section h1 {
  font-size: calc(var(--marpit-root-font-size, 1rem) * 2);
}

The theme author can use :root selector and rem unit with a familiar way, and don't have to take care of the root font size of the integrated service. And developer using Marpit also doesn't have to take care of the font size in the root and the Marpit container.

Resolves #232.

ToDo

• Add test cases