feat: custom component attributes via svelteHTML.ComponentAttributes

[khromov]

Frameworks built on top of Svelte sometimes attach custom namespaced attributes to components (e.g. <MyComponent custom:defer />). Until now these triggered type errors because the attribute isn't part of the component's props.

You can now declare such attributes globally — and have their values type-checked — by augmenting the svelteHTML.ComponentAttributes interface in a d.ts file (e.g. your app.d.ts):

declare namespace svelteHTML {
    interface ComponentAttributes {
        'custom:defer'?: boolean;
        'custom:hydrate:visible'?: { rootMargin?: string };
    }
}

With this in place:

<MyComponent custom:defer custom:hydrate:visible={{ rootMargin: '200px' }} someOtherProp="foo" />

• Only attribute names containing a : are treated as custom component attributes.
• Declared namespaced attributes are validated against their declared types.
• Regular props keep being checked against the component's own prop definitions.
• Any namespaced attribute you don't declare is reported as an error, so typos are caught.

How it works

svelte2tsx routes colon-namespaced attributes on components through a new __sveltets_2_componentAttr helper whose parameter is the augmentable svelteHTML.ComponentAttributes interface (added to both shim sets).

Includes docs, a changeset, and htmlx2jsx + language-server diagnostics fixtures.