Recognize import and image-set strings as CSS URLs

[adamziel]

Finds CSS resource URLs written as quoted strings in @import, image-set(), and -webkit-image-set(), so existing callers can rewrite them through next_url() and set_raw_url().

For this CSS:

@import "theme.css";
.hero {
  background-image: image-set(
    "hero.avif" type("image/avif") 1x,
    url("hero@2x.png") 2x
  );
  content: "not-a-url.png";
}

Before this PR, next_url() returned only hero@2x.png, because it was wrapped in url(). It now returns all three resource URLs, in source order:

theme.css
hero.avif
hero@2x.png

image/avif is a MIME type and not-a-url.png is displayed text; neither is returned as a URL. -webkit-image-set() gets the same treatment as image-set().

Review order: #315 (merged) → #316 → #317 → #314.

URL context is tracked directly in next_url(), without a separate helper or lookahead loop. Comments and malformed string or URL tokens are not returned. The nesting limit counts image-set() functions open at the same time: the 129th is rejected. It does not limit the number of images in one set or separate image sets in the stylesheet.

This is layer 2 of the CSS stack. #315 is merged; this PR now targets trunk. It changes URL recognition only. Streamed input and saved cursors come in the next layer; URL mappings and rewrite_chunk() remain in #314.

Testing

A separate PHP process rewrites a real file containing imports, image sets, and non-URL text. Another file exceeds the nesting limit and must leave the previous output untouched. All three new tests fail against the preceding layer. The full DataLiberation run completed with 2,426 tests and 16 skips. PHPCS passes. The example above was also run against both trunk and this branch to check the before-and-after URL lists.