Support `@container anchored(fallback: ...)` queries

Since the polyfill already has everything wired up related to CSS Anchor Positioning, it would be "relatively easy" to add support for the anchored container queries to it?

That is part of v2 of the spec: https://www.w3.org/TR/css-anchor-position-2/#anchored-container-queries

Are there reasons why it's a bad idea to do it in this polyfill?

Something like

if (!CSS.supports('anchor-name: --a')) {
  await polyfill();                          // covers queries as part of its work
} else if (!CSS.supports('container-type', 'anchored')) {
  await polyfillAnchoredContainerQueries();  // Chromium 125–142
}

Start by reviewing the existing CSS Anchor Positioning polyfill behind the CSS.supports('anchor-name: --a') check, then compare its coverage with the anchored container queries specification. Define the scope for @container anchored(fallback: ...) and the polyfillAnchoredContainerQueries() entry point; done means the supported Chromium range and fallback-query behavior are clearly specified and tested.