Merge #11690

11690: feat: Add an assist for inlining type aliases r=Veykril a=steven-joruk

I'm working towards implementing #10881, but I'd like to get this in first with earlier feedback.

Is `inline_type_alias` a good enough name? I guess the follow up assist would be called `inline_type_alias_into_all_users` based on that.

![valid_inlines](https://user-images.githubusercontent.com/1277939/158020510-fed78b5c-4c7e-46d1-9151-3044a29b9990.gif)

![invalid_inlines](https://user-images.githubusercontent.com/1277939/158020516-8a2deb6d-c6ec-4adf-a15b-c514fc97dc43.gif)



Co-authored-by: Steven Joruk <steven@joruk.com>

Author: bors[bot]