guardrex and scottaddie Improve environment variable coverage (#7092)
Fixes #6760 

The goals are ...

* Surface that env vars aren't added by default with the Generic Host.
* Describe `AddEnvironmentVariables`.
* Deal with env var prefix issue.
* Explain how to set env vars for dev (shoot reader to *Multiple environments* topic for more info). Mention VS, command-line, and VSC config.
* Add VSC example to the *Multiple environments* topic.
* Update the sample app and snippets for the topic.
Latest commit 3ef776e Jun 20, 2018