Allow __error to use a custom layout

[Alekuso]

Describe the problem

Until now, the error page only uses the default layout which limits a lot Sveltekit. This might be a problem for some kind of sites that for example, wants to use a "clear" layout for their error page.

Describe the proposed solution

One of the best solutions I've imagined is to be able to name it like what pages do to get a layout.
For example: __error@customlayout.svelte which will use the __layout-customlayout.svelte layout.

Alternatives considered

No response

Importance

nice to have

Additional Information

No response

[Conduitry]

Duplicate of #4582.