Accept onBeforeScript & onReadyScript as function

This project is very cool!
I'm using the composition of the scenarios in a javascript file and it would be really nice if the `onBeforeScript` and `onReadyScript` in the scenario could accept a function directly other than a filename.

@garris Is that something you could accept as a PR?

Start by tracing how scenario fields are processed, especially the existing onBeforeScript and onReadyScript filename paths when scenarios are composed in a JavaScript file. Confirm how script files are loaded and where scenario validation occurs; the work is done when both fields accept functions directly while existing filename behavior remains supported.

Written by the indexing model from the issue text.