:is is being incorrectly used for ::part selectors

[bakura10]

Hi,

Starting from 1.21.0, Ligthning CSS incorrectly wrap ::part selector into an :is selector which is not valid : https://lightningcss.dev/playground/index.html#%7B%22minify%22%3Atrue%2C%22nesting%22%3Atrue%2C%22customMedia%22%3Atrue%2C%22cssModules%22%3Afalse%2C%22analyzeDependencies%22%3Afalse%2C%22targets%22%3A%7B%22chrome%22%3A6225920%7D%2C%22include%22%3A0%2C%22exclude%22%3A0%2C%22source%22%3A%22.foo%20%7B%5Cn%20%20%26%3A%3Apart(header)%2C%20%26%3A%3Apart(body)%20%7B%5Cn%20%20%20%20display%3A%20none%5Cn%20%20%7D%5Cn%7D%22%2C%22visitorEnabled%22%3Afalse%2C%22visitor%22%3A%22%22%2C%22unusedSymbols%22%3A%5B%5D%2C%22version%22%3A%22local%22%7D

Input:

.foo {
  &::part(header), &::part(body) {
    display: none
  }
}

Incorrect output (1.21.0):

:is(.foo::part(header),.foo::part(body)){display:none}

Correct output (1.20.0):

.foo::part(header),.foo::part(body){display:none}

Thanks :)