Support for the new `:state()` syntax

[KonnorRogers]

With the deprecation of the :--state syntax, I imagine the restrictions around requiring the -- prefix should be removed.

https://groups.google.com/a/chromium.org/g/blink-dev/c/GDmNaxAuCY4/m/WOQD5uRTAQAJ

https://html.spec.whatwg.org/multipage/custom-elements.html#exposing-custom-element-states

It has not been implemented in Chrome yet, but it has been implemented in Safari TP and I believe is being worked on for Firefox.

element-internals-polyfill/src/CustomStateSet.ts

Lines 30 to 32 in f303390

	if (!/^--/.test(state) || typeof state !== 'string') {
	throw new DOMException(`Failed to execute 'add' on 'CustomStateSet': The specified value ${state} must start with '--'.`);
	}

https://codepen.io/dannymoerkerke/pen/KKEyarg

^ Example supporting both syntaxes.

[keithamus]

https://github.com/keithamus/fdgt/blob/main/stateset.js here's a polyfill I use (gets added here) which uses an internal Set to keep track for unsupported browsers, and calls .add in a try/catch (the current standard doesn't validate beyond the first arg being a string so thrown errors would only be from "old" Chrome). Feel free to repurpose as you see fit for this polyfill.