same syntax for custom states and CSS custom selectors

[romainmenke]

• Switch syntax from :state(foo) to :--foo #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?