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/src/module/mod_module.rs

@@ -1,4 +1,4 @@
-use crate::statement::suite::SuiteLevel;
+use crate::statement::suite::SuiteKind;
 use crate::{AsFormat, FormatNodeRule, PyFormatter};
 use ruff_formatter::prelude::hard_line_break;
 use ruff_formatter::{write, Buffer, FormatResult};
@@ -13,7 +13,7 @@ impl FormatNodeRule<ModModule> for FormatModModule {
         write!(
             f,
             [
-                body.format().with_options(SuiteLevel::TopLevel),
+                body.format().with_options(SuiteKind::TopLevel),
                 // Trailing newline at the end of the file
                 hard_line_break()
             ]