Route groups

[posva]

Previous #393

Goal: group common data in a route record. This could be used to:

• implement redirect, route meta, etc without a component too. Maybe allow a file +route.ts or similar

• (folders) Split routes that share a path into different folder structures, and different meta properties (= different layouts, etc)

• (files): add ignored parts to the file name: (thing).vue -> index.vue users(details).vue -> users.vue

• https://kit.svelte.dev/docs/advanced-routing#advanced-layouts-group

• https://docs.solidjs.com/solid-start/building-your-application/routing#renaming-index