Feature request: Add `{parent}` route variable

[kaidesu]

Is your feature request related to a problem? Please describe.

When creating a new collection with an assign parent, it'd be nice to have a {parent} variable for the route pattern to reference the route pattern of the parent.

Additional context

For example, You have a blog page with the following route pattern:

blog

Then you create/assign a posts collection with the following route pattern:

{parent}/{slug}

which would resolve to blog/hello-world. If the blog page was updated with a new title and route, it would automatically be reflected by its children:

news/hello-world

Feature request checklist

• [x] This is a single feature (i.e. not a re-write of the entire CMS)
• [x] The title starts with "Feature request: " and is followed by a clear feature name (Ex: Feature request: add konami code to control panel)
• [x] I have searched for existing issues and to the best of my knowledge this is not a duplicate