Improve docs and usability of EvaluatorBuilder

[translatenix]

• 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