fix(writer)!: use type alias instead of interface for RestProps

[metonym]

Fixes #137

Currently, Sveld creates an Interface type for component props. If the component has RestProps, the interface will extend it. For most cases, this works fine.

type RestProps = SvelteHTMLElements["button"];

export interface ComponentProps extends RestProps { /** ... */ }

However, this can cause a TypeScript error if component props incorrectly extends RestProps.

A very simple example:

type RestProps = SvelteHTMLElements["a"];

export interface ComponentProps extends RestProps {
  tabindex?: string;
}

A TypeScript error occurs because the tabindex on the ComponentProps incorrectly extends that on SvelteHTMLElements["a"] (tabindex can be string | number).

The correct approach would be to use a type alias for the generated component props type.

Note, however, that the below example will result in a TypeScript error for the consumer. This is because tabindex already exists on SvelteHTMLElements["a"] and incorrectly extends the type (string | number vs. string).

type RestProps = SvelteHTMLElements["a"];

export type ComponentProps = RestProps & {
  tabindex?: string;
}

// If using ComponentProps, a TypeScript Error can still occur: Type <X> is not assignable to type 'never'.

The last step is to omit the keys from the component props from RestProps.

type RestProps = SvelteHTMLElements["a"];

type $Props = {
  tabindex?: string;
}

export type ComponentProps = Omit<RestProps, keyof $Props> & $Props;

In summary:

• RestProps can be an intersection of multiple SvelteHTMLElements.
• Create a separate, internal type for component props.
• Create an intersection of RestProps and $Props where keys from $Props are omitted from RestProps.

This is considered a breaking change since the generated component type is exported from the type file.