Add global and nonlocal formatting (#6170)

## Summary

Adds `global` and `nonlocal` formatting, without the "deviation from
black" outlined in the linked issue, which I'll do separately.

See: https://github.com/astral-sh/ruff/issues/4798.

## Test Plan

Added a fixture in the Ruff-specific directory since the Black fixtures
don't seem to cover this.

crates/ruff_python_formatter/src/statement/stmt_nonlocal.rs

@@ -1,12 +1,18 @@
-use crate::{not_yet_implemented, FormatNodeRule, PyFormatter};
-use ruff_formatter::{write, Buffer, FormatResult};
+use ruff_formatter::{format_args, write};
 use ruff_python_ast::StmtNonlocal;
 
+use crate::prelude::*;
+use crate::FormatNodeRule;
+
 #[derive(Default)]
 pub struct FormatStmtNonlocal;
 
 impl FormatNodeRule<StmtNonlocal> for FormatStmtNonlocal {
     fn fmt_fields(&self, item: &StmtNonlocal, f: &mut PyFormatter) -> FormatResult<()> {
-        write!(f, [not_yet_implemented(item)])
+        write!(f, [text("nonlocal"), space()])?;
+
+        f.join_with(format_args![text(","), space()])
+            .entries(item.names.iter().formatted())
+            .finish()
     }
 }