Summary:
Adds support for launching the packager in a new window on Windows.
**Test plan (required)**
Tested that the packager is launched in a completely independent process. This means that exiting the `react-native run-android` command should not close the packager window and it should exit properly when completed even if the packager window is still opened. Pretty much made sure it behaves exactly like on mac.
Also tested that an error in the packager will not close the window immediately to show the error stack trace.
Closes https://github.com/facebook/react-native/pull/7129
Differential Revision: D3240628
Pulled By: mkonicek
fb-gh-sync-id: 007582250536481d2b2376f9a201f8f415fc1080
fbshipit-source-id: 007582250536481d2b2376f9a201f8f415fc1080
Janic Duplessis