Selector: Make selector lists work with `qSA` again

[mgol]

Sizzle 2.3.7 started using CSS.supports( "selector(SELECTOR)" ) before using querySelectorAll on the selector. This was to solve jquery/jquery#5098 - some selectors, like :has(), now had their parameters parsed in a forgiving way, meaning that :has(:fakepseudo) no longer throws but just returns 0 results, breaking that Sizzle mechanism.

A recent spec change made CSS.supports( "selector(SELECTOR)" ) always use non-forgiving parsing, allowing us to use this API for what we've used try-catch before.

To solve the issue on the spec side for older jQuery versions, :has() parameters are no longer using forgiving parsing in the latest spec update but our new mechanism is more future-proof anyway.

However, the Sizzle implementation has a bug - in CSS.supports( "selector(SELECTOR)" ), SELECTOR needs to be a <complex-selector> and not a <complex-selector-list>. Which means that selector lists now skip qSA and go to the Sizzle custom traversal:

CSS.supports("selector(div:valid, span)"); // false
CSS.supports("selector(div:valid)"); // true
CSS.supports("selector(span)"); // true

To solve this, this commit wraps the selector list passed to CSS.supports( "selector(:is(SELECTOR))" ) with :is, making it a single selector again.

See:

• https://w3c.github.io/csswg-drafts/css-conditional-4/#at-supports-ext
• https://w3c.github.io/csswg-drafts/selectors-4/#typedef-complex-selector
• https://w3c.github.io/csswg-drafts/selectors-4/#typedef-complex-selector-list

Fixes jquery/jquery#5177 Ref w3c/csswg-drafts#7280

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