Nicer API around composed members

[LeaVerou]

E.g. currently Props adds side effects to attributeChangedCallback() and has to wrap it manually:

let _attributeChangedCallback = this.prototype.attributeChangedCallback;
this.prototype.attributeChangedCallback = function (name, oldValue, value) {
	this.constructor[props].attributeChanged(this, name, oldValue, value);
	_attributeChangedCallback?.call(this, name, oldValue, value);
};

This is not only clumsy, it also doesn't play nicely with inheritance.

Similarly, it defines observedAttributes like this:

if (!Object.hasOwn(this, "observedAttributes")) {
	Object.defineProperty(this, "observedAttributes", {
		get: () => this[props].observedAttributes,
		configurable: true,
	});
}

which overrides any existing observedAttributes.

Ideally, we should have a better story for plugins that want to…

• add side effects to an existing function
• add values to an existing object or array