Insert empty line between suite and alternative branch after def/class (#12294)

When there is a function or class definition at the end of a suite
followed by the beginning of an alternative block, we have to insert a
single empty line between them.

In the if-else-statement example below, we insert an empty line after
the `foo` in the if-block, but none after the else-block `foo`, since in
the latter case the enclosing suite already adds empty lines.

```python
if sys.version_info >= (3, 10):
    def foo():
        return "new"
else:
    def foo():
        return "old"
class Bar:
    pass
```

To do so, we track whether the current suite is the last one in the
current statement with a new option on the suite kind.

Fixes #12199

---------

Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ruff_python_formatter/src/statement/clause.rs

@@ -380,21 +380,14 @@ pub(crate) struct FormatClauseBody<'a> {
     trailing_comments: &'a [SourceComment],
 }
 
-impl<'a> FormatClauseBody<'a> {
-    #[must_use]
-    pub(crate) fn with_kind(mut self, kind: SuiteKind) -> Self {
-        self.kind = kind;
-        self
-    }
-}
-
 pub(crate) fn clause_body<'a>(
     body: &'a Suite,
+    kind: SuiteKind,
     trailing_comments: &'a [SourceComment],
 ) -> FormatClauseBody<'a> {
     FormatClauseBody {
         body,
-        kind: SuiteKind::default(),
+        kind,
         trailing_comments,
     }
 }