render jupyter percent scripts

[jjallaire]

This PR implements support for rendering Jupyter percent scripts, which are a convention for encoding a notebook within an ordinary script file. For example, consider the following notebook.py:

# %% [markdown]
# Some **markdown**

# %%
1 + 1

You can also include markdown inside a multi-line quote. For example:

# %% [markdown]
"""
Some **markdown**
"""

Jupyter percent scripts are currently supported by a wide variety of tools including VS Code, Visual Studio, PyCharm, Spyder, and Hydrogen. See the Jupytext documentation on percent scripts for additional details: https://jupytext.readthedocs.io/en/latest/formats-scripts.html

This PR implements support for Python, Julia, and R. There is now a claimsFile() method on engines (which replaces claimsExtension()) that is used to determine if a script should be targeted as a percent script.

@cderv It would be great if you could add comparable support for Knitr spin() using the claimsFile() method (which will need to look inside the R script to see if it uses spin() conventions).

@pankgeorg Once this lands you can update your Pluto engine to implement claimsFile() (you'll want to look inside the file to detect whether it is a Pluto notebook)