feat: Replace addProperty with ES6 getters (#96)

feat: Replace addProperty with ES6 getters (#96)

* Replace addProperty with ES6 getters in main module

This allows static code analysis tools to better understand the module
dependencies. For example, you can use VS Code's code navigation feature
to jump to the module definition.

There's a slight difference in implementation, as the getters installed
by addProperty would only call `require` on first invocation and then
replace themselves with the actual module. But since require does
caching itself, I don't see how that would make a big difference in
terms of runtime.

* Replace addProperty with ES6 getters in ConfigFile

* Remove private module util/addProperty