lib, src: add vm.runInDebugContext()

Compiles and executes source code in V8's debugger context.  Provides
a programmatic way to get access to the debug object by executing:

    var Debug = vm.runInDebugContext('Debug');

Fixes #7886.
I did it like that so that you can pass in a function or anything else that has a toString() method.  Also, I don't see why you would want to reject an empty string.