Add empty lines before nested functions and classes (#6206)

## Summary

This PR ensures that if a function or class is the first statement in a
nested suite that _isn't_ a function or class body, we insert a leading
newline.

For example, given:

```python
def f():
    if True:

        def register_type():
            pass
```

We _want_ to preserve the newline, whereas today, we remove it.

Note that this only applies when the function or class doesn't have any
leading comments.

Closes https://github.com/astral-sh/ruff/issues/6066.

crates/ruff_python_formatter/tests/snapshots/black_compatibility@simple_cases__function2.py.snap

@@ -73,22 +73,13 @@ with hmm_but_this_should_get_two_preceding_newlines():
  elif os.name == "nt":
      try:
          import msvcrt
-@@ -45,21 +44,16 @@
-             pass
- 
-     except ImportError:
--
-         def i_should_be_followed_by_only_one_newline():
-             pass
- 
- elif False:
--
+@@ -54,12 +53,10 @@
      class IHopeYouAreHavingALovelyDay:
          def __call__(self):
              print("i_should_be_followed_by_only_one_newline")
 -
  else:
--
+ 
      def foo():
          pass
 -
@@ -146,14 +137,17 @@ elif os.name == "nt":
             pass
 
     except ImportError:
+
         def i_should_be_followed_by_only_one_newline():
             pass
 
 elif False:
+
     class IHopeYouAreHavingALovelyDay:
         def __call__(self):
             print("i_should_be_followed_by_only_one_newline")
 else:
+
     def foo():
         pass
 

crates/ruff_python_formatter/tests/snapshots/format@statement__function.py.snap

@@ -344,6 +344,7 @@ def with_leading_comment():
 # looking from the position of the if
 # Regression test for https://github.com/python/cpython/blob/ad56340b665c5d8ac1f318964f71697bba41acb7/Lib/logging/__init__.py#L253-L260
 if True:
+
     def f1():
         pass  # a
 else:
@@ -351,6 +352,7 @@ else:
 
 # Here it's actually a trailing comment
 if True:
+
     def f2():
         pass
         # a

crates/ruff_python_formatter/tests/snapshots/format@statement__if.py.snap

@@ -203,14 +203,17 @@ def f():
 
 
 if True:
+
     def f():
         pass
         # 1
 elif True:
+
     def f():
         pass
         # 2
 else:
+
     def f():
         pass
         # 3

crates/ruff_python_formatter/tests/snapshots/format@statement__try.py.snap

@@ -263,18 +263,22 @@ except RuntimeError:
     raise
 
 try:
+
     def f():
         pass
         # a
 except:
+
     def f():
         pass
         # b
 else:
+
     def f():
         pass
         # c
 finally:
+
     def f():
         pass
         # d