Make Layout cumulative

[brillout]

See:

• feat: make Layout setting cumulative vike-react#120
• feat: make Layout setting cumulative vike-vue#129

It's quite a neat change as it enables first-class support nested layouts: https://vike.dev/Layout#nested-layouts.

It's somewhat high priority as the docs already assume Layout to be cumulative.

FYI @Blankeos

[Blankeos]

Looking great as always @brillout ! Thanks so much for this!

[brillout]

@Blankeos I think I can speak for @magne4000 that contributions are welcome to vike-solid. It's a fairly simple change (see the main commit linked in the PR descriptions).

[Blankeos]

Yup, I'm working on it actually. Thanks for the links btw.

[magne4000]

Perfect, I was going to say that I would get this done probably next Tuesday, but yes, contributions are welcome!

[Blankeos]

I spent a good few hours on it, but kinda stuck with SolidJS's tricky hydration and rendering. Don't know how to properly render a dynamic array of wrapper components around props.children.

Let me know if you guys have other ideas. Made the PR here anyway: #86.