Support Jupytext-style line comments for REPL evaluation ranges (#15073)

This adds support for detecting line comments in the
[Jupytext](https://jupytext.readthedocs.io/) format. When line comments
such as `# %%` is present, invoking `repl: run` will evaluate the code
between these line comments as a unit.

/cc @rgbkrk 

```py
# %%
# This is my first block
print(1)
print(2)

# %%
# This is my second block
print(3)
```

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Thorsten <thorsten@zed.dev>
Co-authored-by: Thorsten Ball <mrnugget@gmail.com>