[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_db/src/files.rs

@@ -424,9 +424,19 @@ impl File {
 
     /// Returns `true` if the file should be analyzed as a type stub.
     pub fn is_stub(self, db: &dyn Db) -> bool {
-        self.path(db)
-            .extension()
-            .is_some_and(|extension| PySourceType::from_extension(extension).is_stub())
+        self.source_type(db).is_stub()
+    }
+
+    pub fn source_type(self, db: &dyn Db) -> PySourceType {
+        match self.path(db) {
+            FilePath::System(path) => path
+                .extension()
+                .map_or(PySourceType::Python, PySourceType::from_extension),
+            FilePath::Vendored(_) => PySourceType::Stub,
+            FilePath::SystemVirtual(path) => path
+                .extension()
+                .map_or(PySourceType::Python, PySourceType::from_extension),
+        }
     }
 }