New pseudo selectors

[octref]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions

Description of the Change

Add

• :defined: https://developer.mozilla.org/en-US/docs/Web/CSS/:defined
• :focus-visible: https://drafts.csswg.org/selectors-4/#the-focus-visible-pseudo
• :placeholder-shown: https://developer.mozilla.org/en-US/docs/Web/CSS/:placeholder-shown

The pseudo selectors are sourced from https://github.com/mdn/data/blob/master/css/selectors.json

[octref]

@50Wliu Wondering if you can take a look and let me know anything I need to address. Thanks!

[guerrero]

Any update on this?

Regarding the PR it seems that it combines two changes, one to enable custom at-rules and another one to enable support for new pseudo-clasess.

Also it seems that there's no test for the new added pseudo-classes but it seems that the tests are only checking that pseudo-classes work as expected and they don't test which pseudo-classes are explicitly supported