[Bug]: :is and :where selector lists are purged regardless of matching elements

Describe the bug

When purging CSS, if a non-blocklisted selector is present in an :is() or :where() selector (e.g. :is(h1, h2, h3), it is still purged from the resulting CSS.

To Reproduce

1. Create a basic HTML page with an <h2>Lorem ipsum</h2> element.
2. Create a CSS file with a declaration block of :is(h1, h2, h3) { color: red; } (placing * in front of the selector has no effect)
3. Run PurgeCSS on the created CSS file based on the content of the created HTML file.

Expected Behavior

The resulting styles should have :is(h1, h2, h3) { color: red; } and the <h2> should show up in red. Instead the whole declaration block is missing.

Environment

purgecss: 4.1.3 Environment: macOS v12.4, Node.js v18.6.0, npm v8.13.2

Add any other context about the problem here

CSS is minified (Sass -> CSS with compressed output) before being fed to PurgeCSS. Tested with expanded styles, same result.

Running this as an HTML transform on an Eleventy build (v.2.0.0-canary14) which grabs a hardcoded comment line and replaces it with the purged CSS, see this gist. (this makes it a little more difficult to debug)

My workaround is to safelist :is and :where but this causes unused CSS to be kept since the selectors are used across the stylesheet.

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