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.
Actually, can probably add another quick check to make sure it's not an empty string:
`if (typeof code !== 'string' || code.length)`