docs(DEVELOPER.md): added standalone Lua script execution guide

[rhighs]

Summary

• Adds a "Running Standalone Lua Scripts" section to DEVELOPER.md

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}