[Discusson Only] Support added for closing the browser window when ending Live Development

Changed the browser opening so we can get a pid and later use it to close the browser.  Right now it's implemented for Chrome on Windows only.  There's some implementation for Mac and Linux, but need a little modification and testing.

The browser opening/closing code in BrowserControl.js can be re-factored and extended to support other browsers.  For example we can pass arguments to select which browser to open, and send the desired flags.  Also it could be made into its own separate node module.

This could be used for now for development of this extension, but I'm hoping when this extension ultimately replaces Brackets existing Live Dev, it can use a similar implementation to open/close browser instead of using the NativeApp and the native implementation.

[See a related discussion here: https://github.com/adobe/brackets/pull/6852/]