Create ruff_notebook crate (#7039)

## Summary

This PR moves `ruff/jupyter` into its own `ruff_notebook` crate. Beyond
the move itself, there were a few challenges:

1. `ruff_notebook` relies on the source map abstraction. I've moved the
source map into `ruff_diagnostics`, since it doesn't have any
dependencies on its own and is used alongside diagnostics.
2. `ruff_notebook` has a couple tests for end-to-end linting and
autofixing. I had to leave these tests in `ruff` itself.
3. We had code in `ruff/jupyter` that relied on Python lexing, in order
to provide a more targeted error message in the event that a user saves
a `.py` file with a `.ipynb` extension. I removed this in order to avoid
a dependency on the parser, it felt like it wasn't worth retaining just
for that dependency.

## Test Plan

`cargo test`

crates/ruff_notebook/src/lib.rs

@@ -0,0 +1,9 @@
+//! Utils for reading and writing jupyter notebooks
+
+pub use index::*;
+pub use notebook::*;
+pub use schema::*;
+
+mod index;
+mod notebook;
+mod schema;