#1789 Improve scripting feature to allow debugging with breakpoints and step execution

[Hiroshi Miura]

There is no obvious debugging support for the scripting feature in OmegaT 6.1.0 onward.
It is better to have a way to set breakpoints and allow step execution of user scripts.

Detailed architecture proposal is in ADR
https://omegat.readthedocs.io/en/latest/adr/2025006.UserScriptDebugging/