Improve docs and usability of EvaluatorBuilder

- extend, update, and reword docs
  (inspired by pkl-lang.org and Project.pkl docs)
- avoid duplication of docs by referring
  from add/get methods to set methods
- add a few add/get/set methods to
  improve consistency and documentability
- default to empty instead of null
  StackFrameTransformer to improve usability
- do not change builder state in build()
- require non-empty allowedModules
- require null securityManager in setRootDir()
- improve some error messages