Discuss: Deprecate/Remove state command

[tlancina]

The state command was created to manage Cordova plugins before it was supported in Cordova. Cordova now manages plugins and platforms in config.xml: https://cordova.apache.org/docs/en/latest/platform_plugin_versioning_ref/index.html so there is a decent amount of confusion between the Ionic and Cordova behavior. As Cordova platforms and plugins are not an Ionic concern, let's deprecate the state command and document the existing Cordova command workflow, ie plugin add --save.