Add PostCSS root increasing specificity plugin

[yhatt]

To increase the specificity of replaced :root selector, I've added PostCSS root increasing specificity plugin and apply it while packaging style.

Now a below theme shows yellow background to follow the CSS specification.

/* @theme example */
:root { --bg: yellow; }
section { --bg: aqua; background-color: var(--bg); }

The added plugin will add pseudo-class :not(\9) to increase specificity.

This pseudo-class can increase the specificity of a rule. For example, #foo:not(#bar) will match the same element as the simpler #foo, but has a higher specificity.

— https://developer.mozilla.org/en-US/docs/Web/CSS/:not

\9 (hard tab) is not matched into any elements so section:not(\9) has just same meaning as section.

Fix #245.

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By this change, we'll revert the alias added in #246 to keep :root selector in added theme (Update the section size detection to parse :root directly instead of aliasing to section).