Add :has() pseudo-class support - #3755

Summary

This PR adds support for the CSS Selectors Level 4 :has() relational pseudo-class.

It supports relative selector lists and reuses Dompdf's existing selector-to-XPath compiler for selectors inside :has().

Examples:

li:has(> strong:first-child)
article:has(.target)
h2:has(+ p.note)
h2:has(~ p.note)
section:has(> img.hero, > table.featured)

Implementation

• Parses functional pseudo-class arguments, including nested parentheses and quoted content.
• Splits top-level selector lists inside :has().
• Supports relative descendant, child (>), adjacent sibling (+), and subsequent sibling (~) selectors.
• Reuses the existing selector compiler for classes, IDs, attributes, and supported pseudo-classes such as :first-child and :nth-child().
• Rejects nested :has() and pseudo-elements inside :has().
• Converts the relative selectors to XPath predicates evaluated from the candidate element.

Tests

Regression tests cover:

• direct child selectors combined with :first-child;
• descendant selectors;
• adjacent sibling selectors;
• subsequent sibling selectors;
• selector lists;
• nested functional pseudo-classes such as :nth-child();
• compound selectors;
• attribute selectors;
• invalid or nested :has() expressions.