Auto-generate formatter nodes for string parts (#8837)

A follow-up to auto-generate the `FormatNodeRule` implementation for the
string part nodes. This is just a dummy implementation that is
unreachable because it's handled by the parent nodes.

crates/ruff_python_formatter/src/other/string_literal.rs

@@ -0,0 +1,12 @@
+use ruff_python_ast::StringLiteral;
+
+use crate::prelude::*;
+
+#[derive(Default)]
+pub struct FormatStringLiteral;
+
+impl FormatNodeRule<StringLiteral> for FormatStringLiteral {
+    fn fmt_fields(&self, _item: &StringLiteral, _f: &mut PyFormatter) -> FormatResult<()> {
+        unreachable!("Handled inside of `FormatExprStringLiteral`");
+    }
+}