[css] Support CSS :has() pseudo-class

[BenjaminAster]

With the new CSS functional pseudo class :has(), you can start a selector function with >, + or ~ (see specification). However, VSCode shows a syntax error when using these.

.foo:has(> .bar) {
	/* selects all foos that have a bar as a direct child */
	/* ==> syntax error in VSCode */
}

.foo:has(+ .bar) {
	/* selects all foos that are immediately followed by a bar */
	/* ==> syntax error in VSCode */
}

.foo:has(~ .bar) {
	/* selects all foos that are followerd by a bar */
	/* ==> syntax error in VSCode */
}

[taroken6]

Hi @aeschli
I'd like to take a shot at this

[aeschli]

@taroken6 Cool, the code is in https://github.com/microsoft/vscode-css-languageservice.

The code to adopt is around here: https://github.com/microsoft/vscode-css-languageservice/blob/main/src/parser/cssParser.ts#L1409

See the readme for guidance how to run the tests.

[ExE-Boss]

Duplicate of microsoft/vscode-css-languageservice#250