Selector: Make selector lists work with qSA again - #491

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:
```js
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

Co-authored-by: Richard Gibson <richard.gibson@gmail.com>