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.
