feat: make Layout setting cumulative

[brillout]

Main commit: https://github.com/vikejs/vike-react/commit/92f8da478dbfd60af8a89201065811a59d81b2eb (the other commit merely adds a nested layout example).

Context:

• https://github.com/vikejs/vike/discussions/1681
• https://github.com/vikejs/vike-vue/pull/129
• https://vike.dev/Layout#nested-layouts