Export IntrinsicHTMLElements interface

[edoardocavazza]

This PR introduces and exports the IntrinsicHTMLElements interface, which is extended by IntrinsicElements. This approach allows you to declare built-in custom elements without running into interface merging issues.

Example:

declare global {
    namespace preact.JSX {
        interface IntrinsicElements {
            'details': preact.JSX.IntrinsicHTMLElements['details'] | {
                is: 'x-details';
                customProp: boolean;
            }
        }
    }
}

[rschristian]

without running into interface merging issues.

Which issues are those exactly?

[rschristian]

I don't see much harm in it, even if it's rather niche & has some footguns for those that choose to use it (HTMLDetailsAttributes, for example, no longer aligns with the JSX type in your example above).

We probably wouldn't document an example like that so IMO it's fine.

[coveralls]

coverage: 99.536%. remained the same
when pulling 1b8488d on edoardocavazza:extensible-intrinsic-elements
into 68b30fe on preactjs:main.