feat: Added support for declaring router layout

[CatchABus]

This PR adds support for layout per Router.

[ItalyPaleAle]

Thanks for this PR. What is the problem you're trying to solve with this? What does it allow doing that you can't otherwise do?

[CatchABus]

Thanks for this PR. What is the problem you're trying to solve with this? What does it allow doing that you can't otherwise do?

Here is a concept this would help.
I switch between 2 layouts depending on authentication state:

<svelte:component this={$layout}>
  <Router {routes} on:conditionsFailed={conditionsFailed}/>
</svelte:component>

Even though the above does its work, it affects the lifecycle of Router instance causing it to get destroyed and mounted once more if layout is changed.
Of course, I could use 2 Routers in such a case or include layout inside page component itself but then it goes deeper.
For example, I would want to hide both my layout and page, in order to show a spinner indicator or have a great number of layouts.

[CatchABus]

In general, it would be flexible to add components between Router and view component.
Adding parents inside view component itself ruins UI and UX, since import() will have a first time delay and app will seem blank.
If you take a look at svelte-router-spa package, author splits main component into Router and Route.
The benefit of this is you can add route component inside Router as a child or grand child or even grand grand child.

[davidhaley]

@CatchABus, is there a reason that you closed this before it was merged?

[CatchABus]

@CatchABus, is there a reason that you closed this before it was merged?

Hello. Not sure, I probably thought it might not be accepted as a new feature.
I still think it'd be helpful to have it though.