[red-knot] Add 'Format document' to playground (#17217)

## Summary
This is more "because we can" than something we need. 

But since we're already building an "almost IDE" 

## Test Plan



https://github.com/user-attachments/assets/3a4bdad1-ba32-455a-9909-cfeb8caa1b28

crates/ruff_python_formatter/src/db.rs

@@ -0,0 +1,9 @@
+use ruff_db::{files::File, Db as SourceDb, Upcast};
+
+use crate::PyFormatOptions;
+
+#[salsa::db]
+pub trait Db: SourceDb + Upcast<dyn SourceDb> {
+    /// Returns the formatting options
+    fn format_options(&self, file: File) -> PyFormatOptions;
+}