feature: Allow automatically setting `session` per file

[ianhi]

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

I'm hoping to run files with multiple interdependent code blocks. This can be achieved using session but doing so requires adding that to every code block.

Describe the solution you'd like

Add a config option to enable having a default session per source file.

Describe alternatives you've considered

Writing out the session on every code block, which is ok to do - this request is more a quality of life thing.

Additional context