Update Marpit PostCSS root increasing specificity plugin to change selector

[yhatt]

For compatibllity with some CSS parsers, used :not(a) pseudo-class selector instead of :not(\9).

:root {
  color: red;
}

section {
  color: blue;
}

will transform into:

section:not(a) { /* section is always not `a` */
  color: red; /* CSS specificity -> 0-0-2 [win] */
}

section {
  color: blue; /* CSS specificity -> 0-0-1 [lose] */
}

:marpit-root is always added to section selector so :not(a) must always match.

This change will slightly reduce converted CSS size.