Create a global require function

[jasssonpet]

From NativeScript/ios-jsc#84:

The modules are wrapped in a function that defines a local require. However when using the debugger it would be useful to have the require at hand, even if we are not stopped at breakpoint.

We can make that by defining a global require function. It should search from a path relative to the app's root. And expressions such as:

global.require("ui/frame").topmost().ios.navBarVisibility

should easily locate the ui/frame module.