Add CSS :where() selector support to jsar-runtime

[Copilot]

This PR implements full support for the CSS :where() pseudo-class selector in jsar-runtime's CSS parsing engine.

What's Changed

The :where() pseudo-class is a functional CSS selector that takes a selector list as its argument and selects any element that can be selected by one of the selectors in that list. The key characteristic of :where() is that it has zero specificity, making it useful for default styles that can be easily overridden.

Implementation Details

• Added Where pseudo-class type to the FFI enum in PseudoClassType
• Extended Component struct with a selector_list field to handle functional pseudo-classes
• Updated selector parsing logic to properly recognize and process ComponentImpl::Where variants from Servo/Stylo
• Added FFI interface get_component_selector_list() to access inner selector lists
• Bonus: Added :is() support as it uses the same infrastructure

Examples of supported syntax:

/* Basic :where() selector */
:where(h1, h2, h3) {
  margin: 0;
  font-weight: bold;
}

/* :where() with complex selectors */
.container :where(.button, .link) {
  color: blue;
}

/* Element with :where() */
p:where(.highlight, .important) {
  background: yellow;
}

/* Nested :where() selectors */
:where(:where(div), span) {
  display: block;
}

/* Combined with other pseudo-classes */
div:where(.a, .b):hover {
  opacity: 0.8;
}

Test Coverage

Added comprehensive test suite covering:

• ✅ Basic :where() selector parsing
• ✅ Complex selectors with classes and IDs
• ✅ Nested :where() selectors
• ✅ Integration with CSS stylesheet parsing
• ✅ Combination with other pseudo-classes
• ✅ Edge cases and error handling

All existing tests continue to pass, ensuring no breaking changes.

Technical Notes

The implementation leverages Servo/Stylo's existing ComponentImpl::Where support, which was already parsing :where() correctly but wasn't exposed through jsar-runtime's FFI layer. This change makes the functionality available to the JavaScript/TypeScript layer while maintaining the correct CSS specificity semantics.

Fixes #78.

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