Feature request: make code block fence configurable to support nested code blocks

[ngirard]

Hi there,

First, thank you for creating this nice extension!

I've run into an issue where I need to write markdown that includes examples of code blocks. Because mdlab uses triple backticks (```) to delimit its cells, it's not possible to have a triple-backtick code block inside a cell's content, as it prematurely closes the cell.

Current Behavior

A markdown document like this does not parse correctly:

````python
# This is an mdlab cell
def my_function():
  """
  Here is an example of how to use this function in the terminal.
  The following is just documentation text:

  ```bash
  # This inner code block breaks the mdlab parser
  python my_script.py --help
  ```
  """
  print("This cell is for demonstrating documentation.")
````

Proposed Solution

It would be a huge improvement if the code fence string could be made configurable in the VS Code settings. For example, a new setting like mdlab.notebook.fence could be introduced.

• It could default to ``` for backward compatibility.
• Users could change it to ````` or another custom string to avoid conflicts with their content.

This would allow users to easily write documentation or prompts that involve code blocks, which is a very common and important use case for a tool like this.

Thanks again for your great work on this project!