Support for MyST-Style Code Cells for REPL in Markdown

[axtimhaus]

Zed already works very well with executing code cells in markdown documents via a Jupyter kernel using the traditional syntax

```language
some code
```

This enables straightforward editing of Jupyter notebooks in markdown form with Jupytext.
The current preferred format there for markdown notebooks is using MyST syntax (see here) (as it has tons of useful features). This format, however, uses a slightly different syntax based on MyST's directive syntax.

```{code-cell} language
some code
```

Currently, Zed does not recognize these cells with the correct language (neither for syntax highlighting nor for REPL).
It would be very nice if Zed could parse this correctly.