feature: Export env vars for current source and session

[pawamoy]

Is your feature request related to a problem? Please describe.

As we see in the File-tree gallery example, we have to pass the session name to the recursive function. Could be nice to have an exported env var instead for that.

For language which do not allow recursivity, it could be useful to also export the current source.

Describe the solution you'd like

• MARKDOWN_EXEC_CURRENT_SOURCE
• MARKDOWN_EXEC_CURRENT_SESSION

Describe alternatives you've considered

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Additional context

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