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