Enable formatting for Jupyter notebooks (#7749)

## Summary

This PR enables `ruff format` to format Jupyter notebooks.

Most of the work is contained in a new `format_source` method that
formats a generic `SourceKind`, then returns `Some(transformed)` if the
source required formatting, or `None` otherwise.

Closes https://github.com/astral-sh/ruff/issues/7598.

## Test Plan

Ran `cat foo.py | cargo run -p ruff_cli -- format --stdin-filename
Untitled.ipynb`; verified that the console showed a reasonable error:

```console
warning: Failed to read notebook Untitled.ipynb: Expected a Jupyter Notebook, which must be internally stored as JSON, but this file isn't valid JSON: EOF while parsing a value at line 1 column 0
```

Ran `cat Untitled.ipynb | cargo run -p ruff_cli -- format
--stdin-filename Untitled.ipynb`; verified that the JSON output
contained formatted source code.

crates/ruff_diagnostics/src/source_map.rs

@@ -46,10 +46,7 @@ impl SourceMap {
     /// The `output_length` is the length of the transformed string before the
     /// edit is applied.
     pub fn push_start_marker(&mut self, edit: &Edit, output_length: TextSize) {
-        self.0.push(SourceMarker {
-            source: edit.start(),
-            dest: output_length,
-        });
+        self.push_marker(edit.start(), output_length);
     }
 
     /// Push the end marker for an [`Edit`].
@@ -58,16 +55,18 @@ impl SourceMap {
     /// edit has been applied.
     pub fn push_end_marker(&mut self, edit: &Edit, output_length: TextSize) {
         if edit.is_insertion() {
-            self.0.push(SourceMarker {
-                source: edit.start(),
-                dest: output_length,
-            });
+            self.push_marker(edit.start(), output_length);
         } else {
             // Deletion or replacement
-            self.0.push(SourceMarker {
-                source: edit.end(),
-                dest: output_length,
-            });
+            self.push_marker(edit.end(), output_length);
         }
     }
+
+    /// Push a new marker to the sourcemap.
+    pub fn push_marker(&mut self, offset: TextSize, output_length: TextSize) {
+        self.0.push(SourceMarker {
+            source: offset,
+            dest: output_length,
+        });
+    }
 }