Warn about shadowing JS objects

When doing something stupid in a large code base, it takes some time to i.e. realize why

String.fromCharCode(...)

is undefined / does not work as expected. The attached example boils it down to the problem: someone created a String.qml which is imported with the Module and overrides the global JS object. Fun fact is that this worked before for whatever reason.

It would be nice to see some warning on runtime here.