Add exit helper to code mode scripts

[pakrym-oai]

• Summary
  • expose exit through the code mode bridge and module so scripts can stop mid-flight
  • surface the helper in the description documentation
  • add a regression test ensuring exit() terminates execution cleanly
• Testing
  • Not run (not requested)

[cconger]

Unsure if it already has access to close() as its running in a worker context. But fine if we want to have a consistent and named interface to manage this.