Treat functions/classes in blocks as if they're nested (GH-2472)

* Treat functions/classes in blocks as if they're nested

One curveball is that we still want two preceding newlines before blocks
that are probably logically disconnected. In other words:

    if condition:

        def foo():
            return "hi"
                             # <- aside: this is the goal of this commit
    else:

        def foo():
            return "cya"
                             # <- the two newlines spacing here should stay
                             #    since this probably isn't related
    with open("db.json", encoding="utf-8") as f:
        data = f.read()

Unfortunately that means we have to special case specific clause types
instead of just being able to just for a colon leaf. The hack used here
is to check whether we're adding preceding newlines for a standalone or
dependent clause. "Standalone" being a clause that doesn't need another
clause to be valid (eg. if) and vice versa.

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>