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'd prefer the type check go here. e.g.
```javascript
exports.runInDebugContext = function(code) {
  if (typeof code !== 'string')
    throw new Error('code must be a string');
  return binding.runInDebugContext(code);
};