Implement import sorting - #633

This PR implements isort-like import sorting behavior. The intent is to mirror isort when used with the following configuration:

[tool.isort]
profile = "black"
combine_as_imports = true

The main limitations in the current approach are as follows:

1. We still have no way of reconciling conflicting autofix errors. So if you have unused and unsorted imports, Ruff will only fix one of the two in the first pass (probably the unsorted imports, since that block comes first), and you'll have to run Ruff again to fix the other.
2. We don't have any comment-handling, so all comments are removed entirely

For now, this is implemented via an entirely separator visitor, mostly to make the code easier to reason about. I need to benchmark the impact of that choice.

Resolves: #465.