Match :root, html, a:link and a:visited, which have a fixed answer in a PDF #529

Part of #520 (step 9). Changes existing output: decide whether it ships on by default or behind the option from #535.

Measured on gravitypdf at b6a07921, PHP 8.5.4. :root {color:green}, html {color:green} and a:link {color:green} are all dropped; the link keeps the default blue.

Fix.

• a:link → a with an href. a:visited never matches: a PDF has no visited links.
• :root and html → BODY, which mPDF treats as the root. Watch rem: it resolves against default_font_size (src/SizeConverter.php:68-69), not the root element. Mapping html { font-size: 62.5% } onto BODY would shrink text sized in em and leave text sized in rem, where a browser shrinks both. Either feed the root font size into rem, or map every property except FONT-SIZE.
• :hover, :focus, :active, ::before, ::after, ::first-line, ::first-letter: after A descendant rule with a part mPDF cannot match is applied to the element its first parts name #521 these already never match. Dropping them explicitly only keeps their class names out of usedClassNames and maxClassDepth. Low priority.

Tests. One per pseudo-class, and one check that a:hover does not colour the link.

Depends on #521.