Nested layouts to allow embeds

[bothness]

Embed pages should ideally sit beneath the indicator/dataset and area insights pages, but should not have headers and footers, so we need to introduce some kind of layout nesting to allow for these routes to work:

• /datasets/[slug]/embed?{params}
• /areas/[slug]/insights/embed?{params}

Svelte Kit provides documentation for this use case on this page.

The complication in our case is that we'd ideally like to inherit the data from the parent +layout.js but not the layout from the parent +layout.svelte