Blank line between nested and function def in stub files. (#3862)

The idea behind this change is that we stop looking into previous body to determine if there should be a blank before a function or class definition.

Input:

```python
import sys

if sys.version_info > (3, 7):
    class Nested1:
        assignment = 1
        def function_definition(self): ...
    def f1(self) -> str: ...
    class Nested2:
        def function_definition(self): ...
        assignment = 1
    def f2(self) -> str: ...

if sys.version_info > (3, 7):
    def nested1():
        assignment = 1
        def function_definition(self): ...
    def f1(self) -> str: ...
    def nested2():
        def function_definition(self): ...
        assignment = 1
    def f2(self) -> str: ...
```

Stable style
```python
import sys

if sys.version_info > (3, 7):
    class Nested1:
        assignment = 1
        def function_definition(self): ...

    def f1(self) -> str: ...

    class Nested2:
        def function_definition(self): ...
        assignment = 1
    def f2(self) -> str: ...

if sys.version_info > (3, 7):
    def nested1():
        assignment = 1
        def function_definition(self): ...

    def f1(self) -> str: ...
    def nested2():
        def function_definition(self): ...
        assignment = 1
    def f2(self) -> str: ...
```

In the stable formatting, we have a blank line sometimes, not depending on the previous statement on the same level, but on the last (potentially nested) statement in the previous body.

#2783/#3564 fixes this for classes in preview style:

```python
import sys

if sys.version_info > (3, 7):
    class Nested1:
        assignment = 1
        def function_definition(self): ...

    def f1(self) -> str: ...

    class Nested2:
        def function_definition(self): ...
        assignment = 1

    def f2(self) -> str: ...

if sys.version_info > (3, 7):
    def nested1():
        assignment = 1
        def function_definition(self): ...

    def f1(self) -> str: ...
    def nested2():
        def function_definition(self): ...
        assignment = 1
    def f2(self) -> str: ...
```

This PR additionally fixes this for function definitions:

```python
if sys.version_info > (3, 7):
    if sys.platform == "win32":
        assignment = 1
        def function_definition(self): ...

    def f1(self) -> str: ...
    if sys.platform != "win32":
        def function_definition(self): ...
        assignment = 1

    def f2(self) -> str: ...

if sys.version_info > (3, 8):
    if sys.platform == "win32":
        assignment = 1
        def function_definition(self): ...

    class F1: ...
    if sys.platform != "win32":
        def function_definition(self): ...
        assignment = 1

    class F2: ...
```

You can see the effect of this change on typeshed in https://github.com/konstin/typeshed/pull/1/files. As baseline, the preview mode changes without this PR are at https://github.com/konstin/typeshed/pull/2.

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