Enable direct debugging through the CLI

[kmelmon]

Currently, to enable direct debugging, the app developer has to change native code and recompile their app.

This task is tracking the ability to control direct debugging through the react-native run-windows CLI. We can likely model this after #4277, which added an environment variable to control the bundler. The idea is to supply a parameter to the CLI, which turns on the environment variable, and the VS solution can use that variable to define something used in the project.