merge-list-extend should work with generators

[barredterra]

Checklist

• I think this refactoring is useful for everyone (if it's too specific, consider a custom rule)
• I have checked there are no similar issues suggesting the same thing

Description

After running append-to-extend, I get a lot of snippets like the one below. I expected merge-list-extend to further compress this, but so far it only triggers, when a list is passed to extend.

Code Before

my_list = []
my_list.extend(n ** n for n in range(10))

Code After

my_list = [n ** n for n in range(10))]