[selectors] same syntax for custom states and custom selectors

[romainmenke]

• Switch syntax from :state(foo) to :--foo WICG/custom-state-pseudo-class#6
• https://drafts.csswg.org/css-extensions/#custom-selectors

@custom-selector :--heading h1, h2, h3, h4, h5, h6; has been in use for years as a way to declare custom selectors through postcss-custom-selectors

Is it fine that these overlap or does one need to change?

[Loirooriol]

Maybe file an issue in https://github.com/WICG/custom-state-pseudo-class/issues too.

[romainmenke]

Done : WICG/custom-state-pseudo-class#17

[romainmenke]

I think this can be closed after : #4805 (comment) ?