Init markdown with Jupytertext metadata

[myyong]

I ran

jupyter-book myst init --kernel python3 jupyter_book/book_template/markdown.md

which added the following text to the markdown file.

---

jupytext: cell_metadata_filter: -all formats: md:myst text_representation: extension: .md format_name: myst format_version: 0.13 jupytext_version: 1.11.5 kernelspec: display_name: Python 3 (ipykernel) language: python name: python3

Now the markdown file can execute code cells

```{code-cell}
print("1+1=", 1+1)

>1+1= 2